Occurrence of reconnection jets at the dayside magnetopause: Double Star observations

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2007ja012774, 2008 Occurrence of reconnection jets at the dayside magnetopause: Double Star observations L. Trenchi, 1 M. F. Marcucci, 1 G. Pallocchia, 1 G. Consolini, 1 M. B. Bavassano Cattaneo, 1 A. M. Di Lellis, 2 H. Rème, 3 L. Kistler, 4 C. M. Carr, 5 and J. B. Cao 6 Received 27 August 2007; revised 30 January 2008; accepted 11 March 2008; published 27 June [1] We present a statistical study on reconnection occurrence at the dayside magnetopause performed using the Double Star TC1 plasma and magnetic field data. We examined the magnetopause crossings that occurred during the first year of the mission in the LT interval and we identified plasma flows, at the magnetopause or in the boundary layer, with a different velocity with respect to the adjacent magnetosheath. We used the Walén relation to test which of these flows could be generated by magnetic reconnection. For some event we observed opposite-directed reconnection jets, which could be associated with the passage of the X-line near the satellite. We analyzed the occurrence of the reconnection jets and reconnection jet reversals in relation to the magnetosheath parameters, in particular the local Alfvèn Mach number, the plasma b, and the magnetic shear angle. We also studied the positions and velocities of the reconnection jets and jet reversals in relation to the magnetosheath magnetic field clock angle. We found that the observations indicate the presence of a reconnection line hinged near the subsolar point and tilted according to the observed magnetosheath clock angle, consistently with the component merging model. Citation: Trenchi, L., M. F. Marcucci, G. Pallocchia, G. Consolini, M. B. Bavassano Cattaneo, A. M. Di Lellis, H. Rème, L. Kistler, C. M. Carr, and J. B. Cao (2008), Occurrence of reconnection jets at the dayside magnetopause: Double Star observations, J. Geophys. Res., 113,, doi: /2007ja Introduction [2] Magnetic reconnection is one of the most important plasma processes in the universe. Dungey [1961] was the first to propose that the interplanetary magnetic field (IMF) and the geomagnetic field could reconnect, and Paschmann et al. [1979] gave the first observational evidence of the effects of reconnection at the magnetopause. Nevertheless, the physical mechanisms at the base of the reconnection process are still unknown. For example, it is not clear how magnetic reconnection occurs at the terrestrial magnetopause (MP), e.g., whether it takes place preferentially in a quasi-stationary manner along an extended reconnection line or it is intrinsically transient and occurs in reconnection patches randomly distributed on the magnetopause surface. Moreover, it is unknown if local plasma conditions at the magnetopause influence the occurrence of reconnection. Another issue is if reconnection can take place only when the two interacting plasmas have strictly antiparallel mag- 1 Istituto di Fisica dello Spazio Interplanetario, Rome, Italy. 2 AMDL, Rome, Italy. 3 Centre D Etude Spatiale des Rayonnements, Toulouse, France. 4 Space Science Center, University of New Hampshire, Durham, New Hampshire, USA. 5 Blackett Laboratory, Imperial College London, London, UK. 6 Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing, China. Copyright 2008 by the American Geophysical Union /08/2007JA netic fields. In the antiparallel merging model [Crooker, 1979; Luhmann et al., 1984] the magnetospheric and IMF magnetic fields can merge only if they are perfectly antiparallel. If the IMF has a large B y component or is northward, reconnection occurs only at high latitudes, equatorward or tailward of the cusps, respectively. In the component merging model [Gonzales and Mozer, 1974; Sonnerup, 1974] reconnection can occur even if the two magnetic field are not strictly antiparallel: for a not purely southward IMF, reconnection does occur at the subsolar magnetopause, the reconnection line simply tilts with respect to the magnetic equator according to the sign of the IMF B y component. [3] Several statistical studies have been performed in the past to address these questions. Gosling et al. [1990] presented ISEE 2 data relative to 17 events for which reconnection flows at the dayside low latitude boundary layer had velocities opposite-directed with respect to the magnetosheath flow in order to study the influence of the IMF B y component. Scurry et al. [1994] instead selected and studied 58 events at the low-latitude dayside MP for which the reconnection flows had a higher flow speed than the adjacent magnetosheath. Phan et al. [1996] studied 69 MP crossings with high magnetic shear (>45 ) at the lowlatitude dayside MP and quantitatively analyzed the agreement of the flow change across the MP with that predicted for reconnection. More recently, Paschmann et al. [2005] discussed the results of a study of 60 MP crossings, among which 19 show reconnection signatures, observed by Clus- 1of13

2 ter 1 and 3 during a single orbit skimming the near-tail dawnside MP. All these studies report observations of reconnection flows detected at the low-latitude magnetopause for low magnetic shear, which is inconsistent with the antiparallel merging model, even if it must be stressed that the measured magnetic shear could be different from the magnetic shear at the reconnection line. Recently, it has been shown in single event studies that component reconnection actually takes place at the magnetopause by measuring, when the IMF is prevalently northward, the shear angle between the magnetosheath field and the magnetospheric field near the reconnection point [Retinò et al., 2005] or B y dominated [Kim et al., 2002; Pu et al., 2005]. Moreover, Phan et al. [2006], using two spacecraft observations, reported evidence of a tilted X line, hinged in the subsolar region and extending along the dawn flank of the magnetopause to X GSE = 10 R E. Nevertheless, the observations of Scurry et al. [1994] and Phan et al. [1996] are inconsistent with a single X line hinged at the subsolar point, while Gosling et al. [1990] found observational agreement with the component merging predictions. Furthermore, Scurry et al. [1994] and Phan et al. [1996] found that reconnection is more often observed when the magnetosheath b is low, confirming the dependence of the reconnection process on the plasma b, first evidenced by Paschmann et al. [1986]. However, Phan et al. [1996] noted this could be due to the fact that the velocity change across the MP and the motion and thickness of the MP itself depend on the magnetosheath b in such a way that reconnection can be detected more easily for low b values. [4] Here, we use Double Star TC1 plasma and magnetic field data to study reconnection occurrence at the dayside low-latitude magnetopause. The orbit of the TC1 satellite, which is almost equatorial with an apogee of 12.4 R E,is particularly suitable for this study. We analyzed 239 MP crossings, 143 of which show reconnection signatures. Twenty three out of the 143 MP crossings show flow reversals, which indicate that the crossings occur near the reconnection X point. We present the occurrence of the selected reconnection flows and of the flow reversals near the X point as a function of local magnetosheath plasma properties and we illustrate their features according to the orientation of the magnetosheath field. 2. Reconnection Events Selection [5] We use plasma and magnetic field data to show reconnection occurrence at the magnetopause (MP). The plasma data are collected by the Hot Ion Analyzer (HIA) [Rème et al., 2005], a top-hat analyzer that provides three dimensional ion distribution functions over the energy per charge range 5 32,000 ev/e. In this study we use the plasma moments, computed from the distribution functions on board, at 4 s resolution. The magnetic field data are measured by the Fluxgate Magnetometer (FGM) [Carr et al., 2005] and averages over 4 s are used. In the present study all passes through the equatorial dayside and flank magnetopause occurred during the first year of the Double Star mission have been considered. More precisely, we examined all inbound and outbound crossings of the MP between 22 February 2004 and 31 May 2004, when the apogee of the satellite moved from 1300 LT to 1800 LT, and between 12 December 2004 and 29 March 2005 when the apogee of satellite moved from 0600 LT back to 1300 LT. The orbit apogee moved back to 1300 LT in about 13 months due to a precession of the orbit. When reconnection is occurring magnetosheath plasma enters in the magnetosphere and the magnetic tension implied by the sharp bend in the reconnected field lines causes the plasma velocity to change. Therefore, we selected all the magnetopause crossings that show, at the magnetopause proper or in the boundary layer (BL), plasma flows with a different velocity, either in magnitude or direction, with respect to the adjacent magnetosheath. To ascertain quantitatively that the plasma jets are generated by magnetic reconnection we used the Walén test, which should be fulfilled for stationary 1-D rotational discontinuity, and in case of anisotropic plasma [Paschmann et al., 1986, and reference therein] is given by: V 2 V 1 ¼½ð1 a 1 Þ=m 0 r 1 Š 1=2 ½B 2 ð1 a 2 Þ= ð1 a 1 Þ B 1 Š ð1þ where a =(p k p? )m 0 /B 2 is the anisotropy factor calculated from the difference between the parallel and perpendicular pressures. In equation (1) the observed variations of the velocity components across the magnetopause (left-hand side) are compared to the theoretical expectations (righthand side), which contain the variations of the magnetic field components. We performed the Walén test in the spacecraft reference frame. The positive (negative) sign in equation (1) applies when the normal components of V and B have the same (opposite) sign that is, at the dayside magnetopause, the positive (negative) sign applies to observations north (south) of the reconnection site. In equation (1) subscript 2 refers to the magnetospheric side of the boundary, and subscript 1 to the adjacent magnetosheath side. The equation (1) was derived under the assumption that r 1 ð1 a 1 Þ¼r 2 ð1 a 2 Þ [6] This equation tell us that the density of the plasma jets should remain the same as the one of the magnetosheath, as long as the pressure anisotropy factor does not change. For each selected crossing, we chose a reference time interval in the magnetosheath where we computed the averages quantities r 1, a 1, V 1, B 1. For each measurement during the interval under study we evaluated the expected velocity jump DV th, from the right-hand side of the equation (1), and the observed velocity jump DV o = V 2 V 1. Then we computed the angle, Q W, between the two vectors DV th and DV o and the ratio, R W, between jdv th j and jdv o j. The Walén test is meaningless for point measurements in the magnetosheath and we considered the Walén results only for measurements in the MP or in the BL. We establish that the Walén relation is satisfied by a single point measurement when 0.4 < R W < 3, and 0 < Q W <30, for measurement north of the reconnection site, or 150 < Q W < 180, for measurement south of the reconnection site. A reconnection jet is identified when the above mentioned conditions on the Walén relation are satisfied for at least three consecutive single point measurements, that is for a time interval longer than 12 s, and when the average density in this time interval ð2þ 2of13

3 Figure 1. An example of one single MP crossing with reconnection jets. From the top are shown the ion density, velocity and temperature, the magnetic field, the ratio R W, and the angle Q W. The GSE reference system is used. R W and Q W are the ratio and the angle between the variation of velocity predicted by the Walén relation across the magnetopause and the observed one, respectively. When the R W equals unity and Q W equals 0 or 180 the Walén test is perfectly fulfilled for observations north or south of the X line, respectively. The yellow shading indicates the magnetosheath reference interval, while the 11 blue shadings highlight the selected reconnection jets. These are the accelerated plasma flows at the magnetopause or in the boundary layer for which the Walén relation is fulfilled for at least three consecutive data points and with an average density larger than 1 cm 3. is larger than 1 cm 3. The density threshold is used to assure that the plasma considered is not of magnetospheric origin. The requirement of the extent of the reconnection jet helps to avoid accidental fulfillments of the Walén relation which are often observed in the magnetosheath adjacent to the MP crossings for single data points, for which a case by case exclusion must be done. Regarding the requirements on the ratio R W, when it is computed for reconnection flows observed at the magnetopause is usually less than the theoretically expected value 1. Sonnerup et al. [1981] studied 11 crossings and found that R W was always greater than 0.5. Paschmann et al. [2005] reported on reconnection jets characterized by R W as low as 0.5. We used for R W a minimum value of 0.4 to take into account that we required that the Walén relation must hold for at least three consecutive single point measurements. On the other hand, R W values are rarely larger than 1 (one of the 11 crossings of Sonnerup et al. [1981] has R W 1.5). In fact, the condition R W < 3 rarely has effect on the selection (see section 3) and is used as a check on the Walén results Reconnection Jets [7] In Figure 1 a time interval when plasma jets are observed at the magnetopause is shown to illustrate how reconnection jets are selected for the subsequent analysis. From the top are shown the ion density, velocity and temperature, the magnetic field, the ratio R W, and the angle 3of13

4 Figure 2. An example of one single MP crossing with reconnection jet reversals. The format is the same as Figure 1. In this example three jet reversals are selected: the first one on the left is northward directed and the others are southward directed. Q W. The GSE reference system is used for the velocity and the magnetic field. After 0938 UT the satellite is in the magnetosheath: the plasma density and velocity are approximately 10 cm 3 and 200 km/s, respectively, and the three components of the magnetic field are negative. At 0937 UT a complete crossing of the magnetopause occurs: proceeding from right to left, the B z component changes from 15 nt to 40 nt and the B x component changes from 20 nt to about zero, the density decreases, and the temperature slightly increases. Just after the magnetopause crossing, the satellite observes alternatively the BL, where the density assumes values in between the ones of the magnetosheath and of the magnetosphere, and the magnetosphere proper, where the density is 0.2 cm 3 and the temperature is above 2000 ev. We note that the plasma velocity at the magnetopause and in the majority of the BL is greatly enhanced with respect to the adjacent magnetosheath, reaching a speed of about 400 km/s. This acceleration is mainly in the Z direction, that is the direction along which the change in the magnetic field is greatest. This is an indication that the BL formation is due to reconnection. The Walén test is performed: the DV th and DV o are computed using as the magnetosheath reference the interval indicated by the yellow shading, then the R W and Q W are evaluated. The Walén test is meaningless for point measurements in the magnetosheath and the Walén results must be ignored. In the BL and at the magnetopause R W values tend to unity and Q W approaches 180. The plasma flows selected as reconnection jets according to the above mentioned criteria are indicated by the blue shadings. The plasma jets observed around 0910 and 0915 UT are not selected as reconnection jets because the density is below 1 cm 3 and R W is below the threshold values of 0.4, respectively. After the selection procedure, for each reconnection jet the average R W, Q W, density, velocity, and magnetic field are computed Reconnection Jet Reversals [8] In the previous case the reconnection jets come from a reconnection site that is located north of the satellite, in fact DV th and DV o are antiparallel and the Walén relation holds 4of13

5 Figure 3. Scatterplot of R W versus q W for the all the selected jets. The Walén test is perfectly fulfilled when R W equals unity and q W equals zero. The grey dashed line is the linear fit of R W versus q W. with the minus sign. At some magnetopause crossings, reconnection jets moving in the opposite direction are detected within short time intervals. An example of jet reversal is illustrated in Figure 2, which has the same format of Figure 1. A reconnection jet flowing in the Z positive direction, with Q W 0, is observed at the magnetopause between 1402:20 and 1403:04 UT. Then the velocity component V z becomes negative and the plasma accelerates in the Z negative and X positive directions. Two reconnection jets with Q W 180 are identified between 1405:30 and 1406:34 UT. The reversed jets are observed in the BL, where the plasma density is about 4 cm 3 and the magnetic field has reached its magnetospheric value. Reconnection jet reversals have been associated with the satellite being near the reconnection X point [Kim et al., 2002; Retinò etal., 2005; Pu et al., 2005]. We note that in this case the magnetic field shear angle is only Statistical Results [9] In our study we visually inspected approximately 300 inbound and outbound passes across the dayside magnetopause. We studied all the complete crossings of the magnetopause, when the satellite passed from the magnetosheath to the magnetosphere and vice versa. We further required that the magnetic field in the magnetosheath adjacent to the magnetopause was stable to include the MP crossing in the study. Multiple MP crossings may occur during each pass due to the motion of the MP. When the magnetosheath plasma conditions and the orientation of the magnetosheath field do not change for different adjacent MP crossings, only one magnetosheath reference is identified and the MP crossings are considered as one single MP crossing. In total we selected 239 MP crossings with stable magnetosheath magnetic field. Plasma jets which satisfied the Walén relation according to our criteria were found at 143 MP crossings. The total number of selected reconnection jets is 798: in fact, several reconnection jets can be observed at a particular MP crossing, as in the example in Figure 1. In 64 MP crossings the Walén test is never satisfied at the MP or in the boundary layer: we consider these as nonreconnection events. We also found 32 MP crossings for which the Walén test was fulfilled for less than three consecutive measurements; these crossings are excluded from the subsequent analysis. Reconnection jet reversals were observed at 23 of the 143 MP crossings. In summary, 207 MP crossings are considered: 120 MP crossings with reconnection jets (RMP), 23 MP crossings with reconnection jet reversals (RRMP), and 64 crossings with no evidence of reconnection. [10] Regarding the agreement with the Walén relation, in Figure 3 the R W is reported as a function of q W (q W = Q W when 0 < Q W <30 and q W = 180 Q W when 150 < Q W < 180 ) for all the selected reconnection jets and reconnection jet reversals. The grey dashed line is the linear fit of R W versus q W. The majority of reconnection jets have 0.5 < R W < 1, with only 2.4% of reconnection jets having R W >1.5 and 6.5% having R W < 0.5. We note only a very slight tendency of R W to be smaller for larger q W, with q W being comprised in the interval The average R W and Q W values over all reconnection jets are 0.81 (±0.28) and 15.9 (±4.8), respectively. [11] In Figure 4 the positions of the RMP (black dots) and of the RRMP (circled crosses) are shown in GSM latitude and local time. Owing to the satellite orbit and season, they are concentrated at low latitudes ( 40 < Lat < 40 ) and occur mainly in the south for the LT interval and in the north for the LT interval. In the following section we analyze the relationship between the occurrence of the RMP and RRMP and the properties of the magnetosheath adjacent to the MP. 4. Dependence on Magnetosheath Properties [12] We identified two reference intervals, in the magnetosheath adjacent to the MP and in the magnetosphere proper, for each of the 207 MP crossings under study and we computed the average density, velocity, temperature, and magnetic field in such intervals. Subsequently, we computed the local magnetosheath Alfvèn Mach number (M A ) and plasma b, and the shear angle between the magnetosheath and magnetospheric field, f B12. [13] The two bar plots of Figure 5 show the relative contribution (normalized fraction) of RMP (black line) and RRMP (grey shaded) with respect to all magnetopause crossings in each f B12 bin, as a function of f B12. The RMP are observed for 40 < f B12 < 180 and the normalized fraction appears to be almost constant in this interval. We note (not shown) that 7% of the total 207 MP fall in the interval 0 < f B12 <40. Regarding the RRMP, they seem to occur at all f B12. It is important to point out that the jet reversals are observed also for magnetic shear angle as low as [14] The normalized histograms of the local magnetosheath b, binned on a logarithmic scale, observed for the MP crossings without reconnection jets, the RMP and the RRMP are presented in Figures 6a, 6b, and 6c, respectively. The b histogram of MP crossings without reconnection jets is peaked for beta in the range For one MP crossing a b = 61 has been observed. Concerning the RMP and RRMP crossings, both normalized histograms show a maximum for 0.46 < b < 1, while the shape of the distributions looks different being RRMP b histograms 5of13

6 Figure 4. The positions of the RMP (black dots) and of the RRMP (circled points) are shown in GSM latitude and local time. confined to b < In Figure 6d the bar plots of the relative contribution (normalized fraction) of RMP (black line) and RRMP (grey shaded) with respect to all magnetopause crossings in the same b bin, are presented as a function of b. For low value of b the normalized fraction of RMP is 50%, it is around 65% in the beta interval, and then the fraction slightly decreases. The RRMP are concentrated within b interval. These results indicate that the occurrence of RMP and RRMP is higher for lower beta value. The average b for MP crossings without reconnection jets, RMP and RRMP is reported in the first row of Table 1 together with the relative standard error. The average b value for RMP is lower than the average b value for MP crossings without reconnection jets, and the average b value for the RRMP is lower than the average b value for RMP. We tested the statistical significance of the observed agreement among the average values. Assuming the usual 5% significance level for the null hypothesis (statistical agreement), we conclude that these differences are statistically significant. [15] The normalized histograms of the local magnetosheath M A, binned on a logarithmic scale, observed for the MP crossings without reconnection jets, the RMP and the RRMP are presented in Figures 7a, 7b, and 7c, respectively. We note that the RMP and the RRMP seem to be characterized by lower M A values when compared to the MP crossings without reconnection jets. In Figure 7d the bar plots of the relative contribution (normalized fraction) of RMP (black line) and RRMP (grey shaded) with respect to all magnetopause crossings in the corresponding M A bin are presented. The RMP fraction is constant for the M A interval and decreases for M A > The RRMP are observed for < M A < Therefore, it seems that RMP and RRMP occurrence is slightly enhanced for low M A value. In Table 1 (second row) the average M A and the relative standard error for MP crossings without reconnection jets, RMP and RRMP are reported. The average M A is lower for the RMP with respect to MP with no reconnection and for the RRMP with respect to the RMP. We tested the statistical significance of the observed disagreement among the average values. Assuming the usual 5% significance level for the null hypothesis (statistical agreement), we conclude that the difference among M A for MP crossings without jets and for RMP is statistically significant, while the difference of the average M A for RMP and RRMP is not significant. [16] We note that few RRMP are observed when compared with RMP and to the MP with no reconnection signatures. Therefore, we evaluated the cumulative distribution functions (CDFs), i.e., the probability of finding a value of the random variable below a fixed threshold P(X x*), to better study the differences among the three different sets of events in terms of plasma b and Alfvén Mach number. Figure 8 shows the CDFs for the plasma b (Figure 8, top) and the M A (Figure 8, bottom), in the case of MP without reconnection jets (continuous line), RMP (dotted line) and RRMP (dash dotted line). The CDFs for the b and the M A show the same behavior, i.e., a reduction of the range of variability of the plasma b and M A in the case of RMP with respect to the MP with no reconnection signatures. The RRMP seem to be characterized by low b (b < 1) and low Alfvén Mach numbers (M A < 1) also with respect to the RMP. To further investigate if the differences observed in the ranges of variability of the plasma b and Alfvén Mach number are statistically relevant, we performed the Kolmogorov-Smirnov (KS) test. The KS test is a nonparametric test based on the distance between two CDFs which is particularly valid and efficient in the case of small data sets, i.e., when the construction of the probability distribution functions (PDFs) is difficult. One of the most interesting advantages of KS test is that the comparison between two data sets, made to establish if they differ significantly, is performed without any a priori hypothesis about the true distribution of data. In this test the null hypothesis H 0 is represented by the statistical equivalence of the two data sets, and the rejection of this hypothesis is made on the basis of the maximum distance D among the CDFs of the two data sets. In detail, if this distance D exceeds a fixed critical value D c, which depends on the fixed confidence level and on the number of points constituting the two data sets, the null hypothesis is rejected. Figure 5. The two bar plots of the relative contribution (normalized fraction) of RMP (black line) and RRMP (grey shaded) with respect to all magnetopause crossings in the same f B12 bin are presented as a function of f B12 (magnetic shear angle). 6of13

7 Here, we chose as confidence level the usual 5% threshold. To apply the KS test we make use of routines implemented in IGOR Pro 6.01 (Wavemetrics) and based on the work of Birnbaum and Tingey [1951]. As a result of the KS test we conclude that the differences observed among the CDFs of the MP crossings without reconnection data set and the others are statistically significant, while the differences observed between RMP and RRMP, are not statistically significant. This last result is clearly a conclusion of the small dimension (23 points) of the data set relative to the case of reconnection jet reversals. In conclusion we may say that reconnection events are characterized by smaller values of the plasma b and Alfvén Mach number in comparison with the cases of no reconnection, and that the events characterized by jet reversals could be characterized by even lower values of plasma b. 5. Jets Distribution at the Magnetopause [17] In this section we present the positions and the DV o of the reconnection jets and reconnection jet reversals as a function of the local magnetosheath magnetic field clock angle, g, defined as tan 1 (B 1y /B 1z ). In fact, the velocity of a reconnection jet is due to the combined effect of the magnetic tension and magnetosheath convection while using DV o helps to highlight aspects of the reconnection process, especially when the observations are far from the subsolar region. In Figure 9 the projections of the reconnection jets DV o in the ZY GSM plane are shown for 45 < g <45 (Figure 9a), g > 135 and g < 135 (Figure 9b), 135 < g < 45 (Figure 9c), and 45 < g < 135 (Figure 9d). The clock angle sector is indicated in the upper left corners of Figures 9a 9d. The DV o projections are plotted at the positions of the corresponding MP crossings (black dots; all the positions of RMP and RRMP are always reported in Figures 9a 9d). The DV o vectors are blue (red) colored when the Walén relation holds with the plus (minus) sign, that is when the reconnection jets are observed north (south) of the reconnection site. In Figures 9b, 9c, and 9d the reconnection line locations according to the component merging model are also illustrated. In this model, the X line passes through the subsolar point and it is perpendicular to the vector B 2 B 1 [Sonnerup, [1974]]. The X line rotation with respect to the Y GSM axis is larger for smaller clock angles. In Figures 9b, 9c, and 9d the continuous green lines represent the predicted reconnection line locations for g = 180, g = 90 and g = 90, respectively, and the dashed green lines illustrate the maximum rotation of the reconnection line due to the variation of g in each sector. Looking at Figure 9a, we note that only two magnetopause crossings with Figure 6. The normalized histograms of the local magnetosheath b, binned on a logarithmic scale, observed for (a) the MP crossings without reconnection jets, (b) the RMP, and (c) the RRMP are shown. (d) The two bar plots of the relative contribution (normalized fraction) of RMP (black line) and RRMP (grey shaded) with respect to all magnetopause crossings in the same b bin, are presented as a function of b. Table 1. Average Beta and Mach Alfvén Number With the Relative Standard Errors for MP Crossings Without Reconnection Jets, With Reconnection Jets, and With Jet Reversals a No Reconnection Jets (64) RMP (120) RRMP (23) b 4 ± ± ± 0.08 M A 1.5 ± ± ± 0.08 a The number of crossings are shown in brackets. 7of13

8 reconnection jets are observed when 45 < g <45 that is in case of B z positive dominating. For one of the two crossings the clock angle is 44, very close to the sector with B y positive dominating. Considering the sector with g>135 and g < 135 (Figure 9b), that is the case of B z negative dominating, we observe 59 magnetopause crossing with reconnection jets. The clock angles of the reconnection jets are in the range Almost all the reconnection jets positions are consistent with the corresponding reconnection line locations. For example, the reconnection jet north of the reconnection line (blue vector) observed at Y GSM 11 R E and Z GSM 4 R E has a clock angle of 138 and its position is consistent with the dashed reconnection line inclined toward the north at dusk. Only the position of the red reconnection jet at Y GSM 11 R E and Z GSM 4 R E is inconsistent with the predicted reconnection line location. We note that the dipole tilt angle at the time of this observation was 20. The dipole tilt angle is the angle of the north magnetic pole to the Z GSM axis and it is positive when the north magnetic pole is tilted toward the sun. These reconnection jets are directed mainly along Z GSM. The positive (negative) DV oy of the red reconnection jets in the Southern Hemisphere at dawn (dusk) and the negative (positive) DV oy of the blue reconnection jets at dawn (dusk) are due to their positive (negative) clock angles and are in agreement with the corresponding reconnection dashed line inclined anticlockwise (clockwise) with respect to the Y GSM axis. In the 135 < g < 45 interval (Figure 9c), for B y negative dominating, 35 crossings with reconnection jets are found. The clock angles of the reconnection Figure 7. The normalized histograms of the local magnetosheath Mach Alfvén number (M A ), binned on a logarithmic scale, observed for (a) the MP crossings without reconnection jets, (b) the RMP, and (c) the RRMP are shown. (d) The two bars plot of the relative contribution (normalized fraction) of RMP (black line) and RRMP (grey shaded) with respect to all magnetopause crossings in the same M A bin, are presented as a function of M A. Figure 8. Cumulative CDFs for (top) the plasma b and (bottom) the M A in the case of MP without reconnection jets (continuous line) of RMP (dashed line) and of RRMP (dotdashed line). 8of13

9 Figure 9. The reconnection jets velocity variations DV o with respect to the adjacent magnetosheath are projected in the YGSM - ZGSM plane for (a) B z positive dominating, (b) B z negative dominating, (c) B y negative dominating, and (d) B y positive dominating. The DV o are plotted at the positions of the corresponding MP crossing (black dots: in Figures 9a 9d are always reported the positions of all the RMP and RRMP). The blue and red colors refer to reconnection jets located north and south of the X line, respectively. In Figures 9b, 9c, and 9d the continuous green lines represent the reconnection line locations for g = 180, g = 90 and g =90, respectively, and the dashed green lines illustrate the maximum rotation of the reconnection line due to the variation of g in each sector. jets are in the range For the majority of MP crossings in the Northern (Southern) Hemisphere at dusk (dawn) the reconnection jets come from a reconnection site located in the south (north), in agreement with the component merging prediction. We note that the red reconnection jets at Y GSM 6 R E are characterized by large negative dipole tilt angles. The reconnection jets in the vicinity of the predicted reconnection line are directed perpendicular to it, either northward and duskward or southward and dawnward. The jets at Y GSM 12R E and Z GSM 5 R E,far from the expected reconnection line, are mainly directed southward along Z GSM. This is probably due to the rotation of the magnetospheric field, B 2, far from the magnetic equator. Finally, for the 45 < g < 135 interval (B y positive 9of13

10 dominating, Figure 9d), 24 jets are observed. Also in this case, the predicted reconnection line separates the jets according to the velocity direction and to the sign of the Walén relation. The observed clock angles are in the range and all the reconnection jets positions are consistent with the corresponding predicted reconnection line locations. For example, the red reconnection jets south of the reconnection line observed at dusk with 2 R E < Y GSM < 8 R E have small clock angles and their positions are consistent with the dashed reconnection line with the maximum inclination with respect to the Y GSM axis. The reconnection jets in the vicinity of the predicted reconnection line are directed perpendicular to it, either southward and duskward or northward and dawnward. Similarly to the preceding case, the jets observed at Y GSM 12 R E, far from the reconnection line, move southward along Z GSM. Therefore, we conclude that the reconnection jets observations indicate the presence of a reconnection line hinged in the vicinity of the subsolar point inclined according to the sign of the B y in good agreement with the predictions of the component merging model. [18] In Figure 10 the positions and the DV o of the reconnection jet reversals are presented for g > 135 and g < 135 (Figure 10a), 135 < g < 45 (Figure 10b), and 45 < g < 135 (Figure 10c). The format is the same as Figure 9. Moreover, in Table 2 the Y GSM, Z GSM and local magnetosheath clock angle are reported for each of the jet reversals. We recall that reconnection jet reversals are considered to occur near the reconnection X line. We note that no jet reversals are observed in case of B z positive dominating; for g > 135 and g < 135, Figure 10a, 14 jet reversals are observed with respect to 59 RMP; for 135 < g < 45, Figure 10b, three jet reversals are observed with respect to 35 RMP (we note that two out of the three RRMP have clock angles of 133 and 130, very close to the sector with B z negative dominating); finally, for 45 < g < 135, Figure 10c, six jet reversals are observed with respect to 24 RMP. For B y negative dominating, when the satellite explores regions away from the predicted reconnection line, RRMP are found at 8% of the MP crossings with reconnection evidence, while RRMP are observed at more than 23% of the MP crossings with reconnection for B y positive and B z positive dominating sectors, when the satellite is in a more favorable position with respect to the predicted reconnection line. Therefore, the jet reversals are more often observed when the MP crossings occur near to a reconnection line tilted according to the sign of B y as predicted by the component merging model. Moreover the majority of the jet reversals have DV o perpendicular to the corresponding X line. [19] In Figure 11 the line perpendicular to the vector B YZ2 - B YZ1 is reported for each of the reconnection jet reversals (no distinction is made for the magnetosheath clock angle); each line is drawn from the point of Figure 10. Velocity variations DV o and positions of the reconnection jet reversals for (a) B z negative dominating, (b) B y negative dominating, and (c) B y positive dominating. The format is the same as Figure 9. For the sake of clarity only the jet reversals and not all the reconnection jets observed at each MP crossing are reported. 10 of 13

11 Table 2. Positions and Clock Angles of the 23 Reconnection Jet Reversals for the B z Negative, B y Negative, and B y Positive Sectors Y GSM Z GSM Local Clock Angle g > g < < g < < g < observation of the reconnection jet until it intersects the Z GSM axis. We consider this line as an approximation of the reconnection line at the time of observation. According to the component merging model, considering the local magnetic field shear angle as the subsolar IMF clock angle, these reconnection lines should pass in the vicinity of Y GSM = 0 and Z GSM = 0. We found that the computed reconnection lines intersect the Z GSM axis evenly between 6.5 and 7 R E. The reconnection line intersection lies out of the plot for three reconnection jets (labeled 1, 2, and 3), thoroughly discussed in the following. In Figure 12 the distances between the reconnection line intersections with the Z GSM axis and the subsolar point are reported as a function of the dipole tilt angle. The black line in the figure is the linear fit computed discarding the points corresponding to the jets 1, 2, and 3. We note that larger distances from the subsolar point correspond to higher values of the dipole tilt angle. In particular, for positive (negative) tilt angle the reconnection line intersection with the Z GSM axis is southward (northward) of the subsolar point. A possible interpretation of these observations is that the reconnection line location lies preferentially close to the magnetic equator. Actually, Park et al. [2006], using a three-dimensional global MHD simulation of the magnetosphere, found that for a purely southward IMF and a finite dipole tilt angle reconnection occurs near the magnetic equator. On the other hand, a large dipole tilt angle could play a role in the interaction between the magnetosheath flow and the magnetopause so that the stagnation point moves away from the subsolar point. In such case the reconnection line passes in the vicinity of the stagnation point and our observations would show a good quantitative agreement with the component merging model. [20] As far as jets 1 and 2 are concerned, they are the two jets at Y GSM 7 R E and Y GSM 7 R E in Figure 10c with clock angles of 57 and 55, respectively. They are the only two jets at jz GSM j5 R E with small clock angles. The observed reconnection lines orientations can be explained according to two possible frameworks. [21] 1. Points 1 and 2 are located near the high latitude bending of an S-shaped X line, similar to the one reported in Figure 4 of Moore et al. [2002] for the 90 clock angle case. The inclination of the reconnection line observed at the locations of the jets is due to the bending of such reconnection line around the polar cusps. Given the positions of the two jets, the inclination of this S-shaped reconnection line at the subsolar point is lower than the component model prediction. [22] 2. The reconnection lines are straight lines passing through the points of observations but have a large displacement with respect to the subsolar point. Surprisingly, the two observed reconnection lines resemble the two reconnection line branches of the 45 clock angle case of Moore et al. [2002] model; these authors suggest that in this case the actual result at low latitudes could be a single X line that is a compromise between the two branches. Jet 3 is the jet reversal observed at Y GSM 8R E and Z GSM 5 R E in Figure 10b. The corresponding magnetosheath clock angle is 58 and the position of this reconnection jet clearly disagrees with the component merging prediction of a reconnection line tilted toward north (south) at dawn (dusk). [23] Therefore, we conclude that the overall jet reversals observations confirm that reconnection occurrence at the low-latitude dayside magnetopause is consistent with the prediction of the component merging model of a tilted reconnection line hinged in the vicinity of the subsolar point when the B y component is dominating. However, it is important to point out that our observations are limited to 5 R E < Z GSM <5R E due to the orbit of the satellite, that in this study the local clock angle at the magnetosheath is used instead of the IMF clock angle, and that the observed jet reversals are only 23. Moreover, it must be noted that in this study single spacecraft observations are used. However, extended reconnection lines have been observed at the magnetopause taking advantage of simultaneous reconnec- Figure 11. For each reconnection jet reversal the line perpendicular to the vector B YZ2 B YZ1 is drawn from the point of observation to the intersection with the Z GSM axis. These lines are assumed to approximate the reconnection line at the time of observation. 11 of 13

12 Figure 12. The distances between the reconnection line intersections with the Z GSM axis and the subsolar point are reported as a function of the dipole tilt angle. The black line is the linear fit computed discarding the points corresponding to the jets 1, 2, and 3. For positive (negative) tilt angle the reconnection line intersection with the Z GSM axis is southward (northward) of the subsolar point. tion jets observations by two spacecraft [Phan et al., 2000; Phan et al., 2006]. 6. Summary and Discussion [24] We studied all TC1 satellite passes through the dayside magnetopause in the LT interval that occurred during the first year of the Double Star mission (years ). We found 239 clear MP crossings, where 143 MP crossings present reconnection jets as identified by means of the Walén test. For 23 of these crossings we found jet reversals which could be associated with the satellite being near to the X line. We analyzed the occurrence of the RMP and RRMP as a function of the local magnetic shear angle, the plasma b, and the Mach Alfvén number. We found that RMP are characterized by lower values of b in comparison with the cases of MP with no reconnection. The beta dependence of the magnetic reconnection process was first suggest by Sonnerup [1974]. Paschmann et al. [1986] and Phan et al. [1996] found that the Walén relation was better satisfied for crossings with low magnetosheath b. Also Scurry et al. [1994], found that the probability of observing accelerated reconnection jets was greater for low magnetosheath b and it decrease for high b values. However, considering only reconnection flows with higher speed than the adjacent magnetosheath causes the selection of reconnection jets when the magnetic field is strong, and the density in the magnetosheath is small. In fact, as noted by Phan et al. [1996], the theoretical jump of speed across the MP is anticorrelated with the magnetosheath b. Our criteria do not require any increase of speed of the reconnection jets with respect to the adjacent magnetosheath and therefore do not cause the a priori selection of low b RMP. However, Phan et al. [1996] also found that the predicted flow change DV th and the duration of the crossings decrease with increasing b. This could partly explain the worse agreement with the Walén relation. Although for our observations the agreement with the Walén test does not seem to depend on the magnetosheath b, we can not exclude that the shorter duration of the crossings due to high b causes a lower probability of detecting reconnection jets when b is high. Moreover, considering the reconnection jet reversal cases, our observations suggest that the magnetosheath b in vicinity of the diffusion region should be lower than far from it. We found only 23 RRMP and to draw significant conclusions is hard. The b difference between reconnection jets and jet reversals was statistically significant on the base of the hypothesis test, while the Kolmogorov-Smirnov test between the two corresponding Cumulative Distribution functions does not confirm this hypothesis. [25] We found that the RMP are observed for lower M A with respect to MP with no reconnection. These could be due to the fact that the probability of observing reconnection signatures, when considering a large number of crossings, is higher if the reconnection line is stationary and not moving, that is when the M A < 1 at the reconnection site [La Belle- Hamer et al. [1995]]. Actually, the RMP distance from the reconnection line is unknown, but many of the RMP could be located near the reconnection line. The RRMP, which indeed are expected to be near the X point, seem to be characterized by an even lower M A than the RMP, even if this difference is not statistically significant. In particular the seven RRMP observed at the flanks of the MP, where the average M A for MP with no reconnection is 1.97 ± 0.34, have an average M A = 1.20 ± [26] Regarding the local magnetic shear, the RMP and RRMP occurrence does not depend on f B12 value, provided it is f B12 >45. A similar result was found by Gosling et al. [1990], Phan et al. [1996], and Paschmann et al. [2005]. [27] We also studied the velocities and positions of the RMP and RRMP as a function of the magnetosheath clock angle. This could be considered as a proxy of the IMF clock angle. Our observations are consistent with the existence of an extended reconnection line passing in the vicinity of the subsolar point, tilted with respect to the equator according to the sign of the B y component, as predicted by the component reconnection model. This hypothetical reconnection line would pass through the subsolar magnetopause also for clock angles <90. Moreover, our observations show a dependence of the reconnection line location on the dipole tilt angle value. [28] Acknowledgments. This work was supported by the Agenzia Spaziale Italiana (ASI contract I/035/05/0). The authors thank both the reviewers for their valuable suggestions. [29] Wolfgang Baumjohann thanks Goetz Paschmann and another reviewer for their assistance in evaluating this paper. References Birnbaum, Z. W., and F. H. Tingey (1951), One-sided confidence contours for probability distribution functions, Ann. Math. Stat., 22, of 13

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Kistler, Space Science Center, Science and Engineering Research Center, University of New Hampshire, Durham, NH 03824, USA. (lynn.kistler@unh.edu) H. Rème, Centre D Etude Spatiale des Rayonnements, 9 Avenue du Colonel Roche, B.P. 4346, F Toulouse CEDEX 4, France. (henri.reme@cesr.fr) 13 of 13