Periodic emergence of multicomposition cold ions modulated by geomagnetic field line oscillations in the near-earth magnetosphere

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003ja010141, 2004 Periodic emergence of multicomposition cold ions modulated by geomagnetic field line oscillations in the near-earth magnetosphere M. Hirahara Department of Physics, Rikkyo University, Toshima-ku, Tokyo, Japan K. Seki Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Japan Y. Saito and T. Mukai Institute of Space and Astronautical Science, Kanagawa, Japan Received 15 July 2003; revised 9 October 2003; accepted 19 November 2003; published 18 March [1] In the equatorial magnetosphere during perigee passing, the Geotail spacecraft sometimes observes periodic enhancements of cold ion flux embedded in the energetic ion component. In particular, the dawnside and duskside regions, the periodic signatures indicate not only oscillating bulk motions of both energetic and cold ion components in the inner magnetosphere but also the existence of multiple ion components at thermal energies, namely, singly-charged hydrogen, helium, and oxygen of ionospheric origin. Although these cold components would usually be invisible due to the low energy limit of the instrument and the positive potential of the spacecraft, the excitation of Pc 5 pulsations often causes the periodic emergence of the hidden cold ions after their escape from the ionosphere without further significant energization. The distinctive multicomponent ion signatures can be seen frequently from 1999 to 2002, near solar maximum, but not earlier in the Geotail mission, in in the early rising phase of the solar cycle. It is probable that outflow processes of the ionospheric ions are active at times of high solar activity. A significant fraction of the near-earth magnetospheric ion population is dominated by ionospheric ions, as shown in the density calculations for the separate ion species. INDEX TERMS: 2730 Magnetospheric Physics: Magnetosphere inner; 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions; 2752 Magnetospheric Physics: MHD waves and instabilities; 2451 Ionosphere: Particle acceleration; KEYWORDS: pulsation, Pc 5, multiple composition, cold component, ionospheric origin, periodic emergence Citation: Hirahara, M., K. Seki, Y. Saito, and T. Mukai (2004), Periodic emergence of multicomposition cold ions modulated by geomagnetic field line oscillations in the near-earth magnetosphere, J. Geophys. Res., 109,, doi: /2003ja Introduction [2] Fifteen years have passed since Chappell et al. [1987] proposed that the terrestrial-origin plasmas could fill the Earth s magnetosphere via several types of plasma outflow process from the polar ionosphere. Because the energies of the outflowing plasmas are generally low (a few tens of ev), it is difficult to detect those components with plasma instruments on board spacecraft even after their escape from the Earth s gravity due to electromagnetic acceleration. The electrostatic potential of spacecraft exposed to the solar ultraviolet radiation in the near-earth space is typically several tens of volts positive, which is caused by photoelectron emission from the spacecraft surface. Copyright 2004 by the American Geophysical Union /04/2003JA [3] In the past three decades, a number of spacecraft, for instance, GEOS, DE, ISEE, AMPTE, Viking, Akebono, Freja, FAST, etc, explored the terrestrial magnetosphere by using measurement techniques of plasma energy analyzers and ion mass spectrometers for identifying the origins and the source regions of the space plasma distributed near the Earth. It is well known that there are two kinds of origin regarding the magnetospheric plasma. One is the solar corona, which is the source of the solar wind, and another is the Earth s atmosphere. The results of numerous in-situ observations have revealed that a significant amount of the magnetospheric ions is supplied from the Earth s atmosphere, especially via the plasmasphere inside L = 3 6 and the polar ionosphere directly mapped to the outer magnetospheric regions. As tracers of the ionospheric- and plasmaspheric-origin ions, singly charged helium and oxygen (He + and O + ) are most useful 1of8

2 because these ions are not contained in the solar wind plasma [e.g., Young et al., 1982]. [4] Recently, the Polar satellite, launched in February of 1996, observed terrestrial ions with thermal energies less than a few tens of ev, which are escaping from the polar ionosphere to the geomagnetic tail through the Polar apogee (9 R E ) at the high geomagnetic latitude [Moore et al., 1997]. These data clearly show that the ion fountain phenomena seen in the polar region supply the atmospheric ions to the magnetosphere along geomagnetic field lines. Before the outflow from the polar ionosphere, the ion energies could be less than several ev for O + because the escape velocity from the Earth s gravity is 11 km/sec. The ionosphericorigin ions, however, are frequently accelerated and/or heated up to several kev by many types of electrostatic or electromagnetic energization mechanisms in the topside ionosphere during their outflowing processes. From this point of view, the finding of thermal ions with energies of several ev by Polar is one of the most surprising results made by recent spacecraft mission exploring the near-earth space. [5] The Geotail spacecraft brings another kind of important observations regarding the cold ion population in the magnetosphere. Seki et al. [2003] present that the cold ions of ionospheric origin are often observed also in the equatorial magnetosphere when the spacecraft entered the Earth s shadow. The geomagnetic field lines at high geomagnetic latitudes map the ionosphere to the equatorial regions of the magnetosphere, which means that the ions escaping from the polar ionosphere arrive all the way at the equatorial magnetosphere. The equatorial magnetosphere is a sort of plasma reservoir and also an accelerator of the magnetospheric plasmas. In particular, the nightside of the equatorial magnetosphere is called the plasma sheet, where the hot plasma component is stored after undergoing some energization processes. It is well established observationally and theoretically that this region of the magnetosphere is the source of energetic auroral particles precipitating into the polar ionosphere. Although the plasma sheet ions typically gain energies of several kev due to some dominant energization processes during the transport to and in the plasma sheet, it is becoming evident that there is also another ion component with extremely low energies (below 10 ev) on the basis of the Geotail results. Matsui et al. [1999] also reported Geotail observations concerning the existence of cold dense plasma with densities and energies more than 2/cm 3 and 32 ev, respectively. They concluded that these phenomena are a result of leakage of the plasmaspheric ions and a partial contribution of the direct supply of the ionospheric ions. [6] In this paper we report recent observational results obtained by the Geotail spacecraft near the perigee of 9 R E in the equatorial magnetosphere, which are obtained by the magnetic field (MGF) instrument [Kokubun et al., 1994] and the low-energy particle (LEP) instrument [Mukai et al., 1994]. Our results are essential and unique for revealing the significant contribution of the ionosphere to the magnetosphere because the ion species could be identified on the basis of the natural mass spectroscopic effect. As mentioned above, it is difficult to detect thermal ions by spacecraft that is positively charged due to solar radiation. It is easily expected that the mass discrimination is also difficult especially for these ion components in the magnetosphere. On the other hand, the ion species are definitive clues for determination of the origin of the magnetospheric plasma. The instrumental capabilities of Geotail and the other spacecraft are originally inadequate to obtain the indispensable information. As far as the Polar satellite, an active instrument is used to neutralize the satellite potential. In order to overcome these tough problems on the Geotail case without regular operation of this type of instrument, we take notice of the natural velocity filter effect that gives each ion species an energy propotional to the mass. [7] Near the Geotail perigee, the geomagnetic field line oscillations, so-called the Pc 5 pulsations, are frequently observed. Before our works using the Geotail results, Kokubun et al. [1977] studied the relation of the plasma particle flux modulation with the Pc 5 waves which cause the plasma bulk oscillations because the magnetospheric plasmas are frozen in the geomagnetic fields. The recent plasma instruments with high sensitivity and wide field-ofview could realize more evident and accurate measurements for the periodic flux enhancements as reported by Sakurai et al. [1999] and species identification of the cold ions associated with the Pc 5 events as discussed in this paper. [8] When the amplitudes of the pulsations become large, the bulk velocities of the cold plasma oscillating with the local magnetic field lines are also large and their energies are high enough to be detected on Geotail when it is positively charged. As a result, we have succeeded in identifying the ion species of the cold components and calculating the density of each component. This provides valuable information on the ionospheric contribution to the magnetospheric plasma population. 2. Observations [9] We surveyed the Geotail LEP data for more than seven years (1 May 1995 to 21 June 2002) to investigate the properties of periodic plasma bulk oscillations accompanied by ion flux enhancements at low energies. First of all, we show several typical features of the oscillation events by using two examples. [10] Figure 1 presents an example of the plasma bulk oscillations associated with the Pc 5 pulsations observed in the dawnside region on 18 June From the top, three components of the local magnetic field and the plasma bulk Figure 1. First example of the Pc 5 pulsation and plasma bulk oscillation observed on 18 June From the top, time variations of three magnetic field components measured by MGF, three plasma bulk velocity components and its density by LEP are shown. The color panels are energy-time spectrograms of energy fluxes of electrons and ions versus universal time in color code. The ion spectrograms sorted into four flow (duskward, sunward, dawnward, tailward) directions are also shown above abscissa labels of universal time and spacecraft position in the SM coordinates. The bottom panels are expanded ion spectrograms for detailed presentation of the multiple ion signatures. The Geotail orbit during this interval is also plotted by a red curve. 2of8

3 Figure 1. 3of8

4 velocities are plotted, respectively. The third panel shows the ion density, and the fourth and fifth panels are color spectrograms of energy fluxes of electrons and ions with energies between 30 evand 40 kev versus universal time (UT). It should be noted that the low energy limits of the electron and ion measurements are usually not low enough to detect the thermal components in the magnetosphere. The next four color plots are direction-sorted energy-time spectrograms of ions. In these series of plots, the flow directions of ions are divided into four sectors, namely, duskward, sunward, dawnward, and tailward. It is seen that the fluxes of ion components with energies of a few tens of ev up to several kev are periodically enhanced during UT, UT, and UT. As well as the lowenergy ion fluxes, the high-energy (more than 1 kev) ion and electron fluxes seem to be modulated with the similar frequency, as seen especially during UT. These properties are common in the events observed by Geotail [Sakurai et al., 1999]. The high-energy diffuse components of electrons and ions correspond to the hot plasmas energized in the equatorial magnetosphere, namely, in the plasma sheet. [11] The period of these flux enhancements is about 7 minutes for every signature and corresponds to that of typical Pc 5 pulsations. Also in the magnetic field data, the wavy variations are found to have the same periodicity. More significant variations with sinusoidal form are noticed in the ion bulk velocities in the second panel from the top, which are in a good correlation with the periodic signatures seen in the energy-time spectrograms. [12] The flux enhancements in the duskward and tailward directions are in phase, as seen in the expanded panels at bottom. The sunward and dawnward fluxes have the same characteristic and are in antiphase with the oscillations of duskward/tailward fluxes. This correspondence of the flow directions means that the plasma bulk motion is roughly of a linear oscillation mode. It should be noted that the large velocity oscillations of amplitudes up to ±100 km/s overlaid on the almost stagnant (20 30 km/s) ion components. The ion density also shows a series of large fluctuations associated with the periodic enhancements of cold ion fluxes. [13] A remarkable feature of these sequential events is that the multiple discrete signatures are observed at different energies during the intervals of enhanced plasma bulk velocities. For example, the flux enhancement in the tailward direction during UT consists of two discrete components, whose peak energies are 2 kev and a few hundreds of ev, respectively around 0157:30 UT. In addition, a third component at energies between these two components is sometime seen although this component is fainter and not as readily recognized in the spectrograms. For instance, this middle component is observed during UT in the dawnward direction. The ion densities are also fluctuating, sometimes reaching 0.9/cm 3 and being accompanied by the appearance of the multiple signatures and velocity enhancements. In the event in Figure 1, the signatures at the highest and the lowest energies have similar fluxes while the fluxes of the middle components are always the lowest of these three. [14] The triple signature is also noticable in the next example shown in Figure 2 although the duration is shorter than the previous event in Figure 1. The flux enhancement particularly during UT has three clearly discrete components. The three-dimensional velocity distribution of the triple ion signatures is shown in Figure 3, in which all ion components plotted are assumed to be H +. In the plot, we can see three beam-like ion signatures almost in the same flow direction, which has a large angle to the local magnetic field. It is, therefore, concluded that the flow velocities perpendicular to the field are dominant over the parallel velocities. [15] If we assume that the triple signatures are the lowenergy components of H +,He +, and O +, respectively, the perpendicular velocities for these ion components would be nearly equal. These three ion species are typical in the Earth s magnetosphere, and the existence of He + and O + indicates that the cold ion components originate from the plasmasphere and/or the ionosphere because these ions are not present in the solar wind. On the other hand, H + ions are not used as ionospheric ion tracer because they are the most dominant component in the solar wind. [16] For this event, we have calculated the velocities and densities of these triple components to identify the ion species. From three-dimensional velocity distribution data of 12-sec sampling, we picked up the ion counts separately for every discrete ion component and then carried out the moment calculations using the picked-up counts on the assumption of the three ion species. Figure 4 presents the three velocity components and the densities of the three discrete signatures, and shows that the perpendicular velocities for three ion components were almost equal. The parallel velocities were much smaller than the perpendicular ones, and they were often different from each other, in contrast to the perpendicular velocities. The result suggests that our ion species assumption is correct under the socalled frozen-in condition that is applicable to magnetospheric plasmas driven by the convection electric field. [17] In this event, the H + ions have the largest density, and the O + ions the next dominant density. The lowestdensity component is He +, as expected from the energy-time spectrograms in Figure 2. These density properties are similar to those of the other discrete ion signatures observed by the Akebono in the polar magnetosphere [Kaya et al., 1990; Hirahara et al., 1997] and Geotail in the magnetotail [Hirahara et al., 1996; Seki et al., 1999]. The total density of the discrete signatures consisting of three ion species reaches 0.7cm 3 around 2103 UT, which means that the density of the low-energy ions modulated by Pc 5 is comparable to that of the higher-energy diffuse components, being consistent with the density plot of Figure 2. This indicates that the cold ion components hidden in the magnetosphere could be equal in density to that of the energetic ions or dominant in all components. [18] The left of Figure 5 shows the Geotail orbit by green dots at every 10 minutes during the period from 1 May 1995 to 21 June The oscillation events accompanied by the enhancements of cold ion fluxes, surveyed by visual inspection and plotted in the right of Figure 5, were found mainly around the dawnside perigee. The multiple (double or triple) ion signatures denoted by blue curves in the right of Figure 5 show a similar spatial distribution as the single-type events denoted by red curves. This spatial distribution obtained from the Geotail data is similar to that presented by Takahashi et al. [2002]. It is probable that the locations of most of events are 4of8

5 Figure 2. Second example on 21 April The panel format is the same as that of Figure 1. 5of8

6 Figure 3. An example of three-dimensional velocity distribution for the event of 21 April just inside the magnetopause because the magnetopause crossings by Geotail are frequently observed in the vicinity of these events (not shown here). The spatial distribution of the Pc 5 pulsations shown in Figure 5 implies that the excitation energy of the pulsations is from the interaction between two distinguishable regions, i.e., the magnetosheath and the magnetosphere, as proposed by Dungey and Southwood [1970]. [19] From the year of 1998, the solar activity gradually increases and reaches the maximum at the end of As suggested by previous works, the ionospheric-origin ion fluxes detected in the Earth s polar magnetosphere [Yau et al., 1985; Chandler et al., 1991; Cully et al., 2003] and equatorial magnetosphere [Young et al., 1982] have a tendency to increase with increasing solar activity. In all events of periodic flux enhancements seen in the cold ion components, the multiple discrete signatures consisting of ionospheric-origin heavy ions could be observed frequently from the beginning of 1999 to near the end of the data interval surveyed in this paper (May 2002). Averaged annually during these years, the durations of the events in which the multiple ion signatures can be found are corresponding to 30 70% of the total intervals of the periodic ion signatures. The occurrence tendency obtained from the seven-year survey of the Geotail data is similar to the statistical results shown in the above studies although the previous observations in similar regions were made mainly for more energetic ions than thermal/cold ions. Unless the large-amplitude Pc 5 pulsations occur, the multiple ion components would not be observed because of the low energy limit of the instrument and the positive potential of spacecraft. The results from the Geotail, therefore, indicate that cold ions exist in the equatorial magnetosphere that are devoid of significant energization in the plasma sheet, as recently proposed by Seki et al. [2003]. 3. Summary and Discussion [20] The properties of periodic cold ions in the near-earth plasma sheet observed by Geotail are summarized as follows. Figure 4. Three-dimensional velocities and densities of the triple ion signatures whose ion species are assumed H +, He +, and O + from the lowest component for the second event of 21 April of8

7 Figure 5. Spatial distribution of the Geotail orbits (green dots at every 10 minutes) and events of cold ion flux enhancements with single component (red curves) and multiple components (blue curves) in the SM coordinates. [21] 1. The plasma bulk oscillations consisting of multiple (two or three) ion components are often observed in the equatorial magnetosphere in the near-earth dawn and dusk sectors by Geotail. All events found in the Geotail database are near the perigee of 9 RE. Pc 5 pulsations are associated with these periodic discrete signatures of cold ion components. [22] 2. Assuming that the ion components are O+, He+, and H+ in the 21 April event, the perpendicular velocities to the magnetic field are almost the same for the three ion components. This suggests the ionospheric origin for these cold ions. It should be noted that two- or three-component ions of ionospheric origin consisting of discrete (i.e., energydispersed or velocity-filtered) signatures were previously reported in other Akebono and Geotail observations. [23] 3. In the 21 April event, the H+ density is the highest among the multiple signatures, and the O+ density is the next highest. The He+ ions had the lowest density. The density ratios among the three ion species are similar to the other Akebono and Geotail events of velocity-filtered ions of ionospheric origin. [24] 4. A statistical study is conducted for the period from 1995 to From 1997 to 2000, both the geomagnetic activity seen in the Kp index and the solar activity represented by sunspot number gradually increased. The multiple ion components are observed more often in higher-solar activity years of than in the low-solar activity period from 1995 to These properties are also similar to those found in other phenomena such as previous observations in the polar magnetosphere, the lobe/mantle, the flankside magnetopause, and the plasma sheet. [25] These results indicate that not only the origin but also the escape/outflow processes of the discrete ion signatures presented here could be the same or identical to the velocity-filtered ion signatures observed in various regions of the magnetosphere. On the other hand, the original energies of the discrete ion signatures are lower than those of the velocity-filtered signatures in the other studies. The recent Geotail results also suggest that a significant amount of the ionospheric-origin ions exists in the plasma sheet as unenergized cold components. We should investigate more extensively the observations of cold ion components in the magnetosphere in order to reveal the presence or absence of energization during their transport and identify the escape/ outflow routes of the terrestrial-origin ions. [26] Acknowledgments. We thank the Geotail-MGF team for the data distribution. We are also grateful to the referee for word editing on our manuscript. [27] Lou-ChuangLee thanks Andrew W. Yau and Susumu Kokubun for their assistance in evaluating this paper. References Chandler, M. O., J. H. Waite Jr., and T. E. Moore (1991), Observations of polar ion outflows, J. Geophys. Res., 96, Chappell, C. R., T. E. Moore, and J. H. Waite Jr. (1987), The ionosphere as a fully adequate source of plasma for the Earth s magnetosphere, J. Geophys. Res., 92, of 8

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