Plasma irregularities adjacent to auroral patches in the postmidnight sector

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2010ja015319, 2010 Plasma irregularities adjacent to auroral patches in the postmidnight sector K. Hosokawa, 1 T. Motoba, 2 A. S. Yukimatu, 2 S. E. Milan, 3 M. Lester, 3 A. Kadokura, 2 N. Sato, 2 and G. Bjornsson 4 Received 29 January 2010; revised 28 April 2010; accepted 29 April 2010; published 2 September [1] We demonstrate a close association between decameter scale plasma irregularities in the E region ionosphere and auroral patches in the postmidnight sector. In September 2009, campaign based measurements of the aurora were conducted in Iceland with a white light all sky camera (ASC) at Tjörnes (66.20 N, W) and the SuperDARN radar at þykkvibaer (63.77 N, W). On one night during the campaign period, the ASC observed the successive passage of auroral patches in the postmidnight sector after a small substorm like activity. The patches were drifting predominantly eastward across the field of view of the ASC with a speed of approximately m s 1, which is consistent with the sunward convection in the postmidnight westward electrojet. The simultaneous radar measurements recorded strong radar backscatter echoes (>15 db) within the gaps between adjacent auroral patches, while such echoes were not observed or were very weak in the region of the aurora. The Doppler velocity estimation showed that the electric field was clearly reduced within the patches, which was probably the result of the enhanced conductance associated with auroral precipitation. Thus, this reduction in the electric field suppressed the generation of irregularities (i.e., radar echoes) in the regions of auroral patches. This suggests that the conductance enhancement associated with precipitating electrons not only modified the electric field within the aurora but also affected the generation of small scale plasma structures in the vicinity of the patch type optical auroral forms. Citation: Hosokawa, K., T. Motoba, A. S. Yukimatu, S. E. Milan, M. Lester, A. Kadokura, N. Sato, and G. Bjornsson (2010), Plasma irregularities adjacent to auroral patches in the postmidnight sector, J. Geophys. Res., 115,, doi: /2010ja Introduction [2] Small scale plasma structures, or so called fieldaligned plasma irregularities, in the auroral E region ionosphere, whose scale size ranges from a few meters to a few tens of meters, have mainly been studied using groundbased radar measurements. In particular, the spatial collocation of optical auroral forms and plasma irregularities (i.e., radar aurora) in the auroral E region was investigated extensively using coherent radars at VHF frequencies [Balsley et al., 1973; Greenwald et al., 1973; Tsunoda et al., 1974]. These studies in the 1970s demonstrated that, rather than being co located, backscatter echoes from meter scale irregularities were observed to be adjacent to auroral arcs. Since the 1990s, the establishment of the Super Dual 1 Department of Information and Communication Engineering, University of Electro Communications, Tokyo, Japan. 2 National Institute of Polar Research, Tokyo, Japan. 3 Department of Physics and Astronomy, University of Leicester, Leicester, UK. 4 Science Institute, University of Iceland, Reykjavik, Iceland. Copyright 2010 by the American Geophysical Union /10/2010JA Auroral Radar Network (SuperDARN) [Greenwald et al., 1995; Chisham et al., 2007], which is a network of coherent HF radars in the polar region, has made it possible to observe the radar echoes caused by the Bragg scatter from decameter scale irregularities in the auroral E region [Milan and Lester, 1998, 2001]. [3] HF radar backscatter echoes from the E region fall into two categories, type I and type II, depending on the mechanism through which the ionospheric irregularities responsible for scattering the radio wave are produced [Fejer and Kelley, 1980]. In the E region the ions are collisionally bound to the neutral atmosphere since the ion neutral collision frequency is sufficiently high, whereas the electrons move with an E B drift. This will allow the two stream instability [Farley, 1963; Buneman, 1963] to produce irregularities, which are observed as type I echoes. The Doppler velocity of type I echoes is constrained to be near the local ion acoustic speed C s ( m s 1 at the E region altitudes); thus, type I echoes tend to require some minimum electric field (velocity threshold) for excitation [Nielsen and Schlegel, 1983]. A second plasma instability, the gradient drift instability [Ossakow and Chaturvedi, 1979], is operative when the electron density gradients 1of10

2 exist and generates type II echoes in HF radar backscatter. There is no specific velocity threshold for the excitation of the gradient drift instability. The irregularities move with the background E B drift (i.e., electron flow speed in the E region); thus, type II echoes can be used to estimate the convection speed. As is described by Milan et al. [1997] in detail, the SuperDARN HF radars can detect both E and F region irregularities. In this paper, however, we focus on the HF radar backscatter from E region plasma irregularities. [4] Considerable work has been done by using the SuperDARN data in the past decade concerning the relationship between HF radar backscatter in the E region and the so called discrete auroral arcs, which are longitudinally extended curtain like auroral forms [Milan et al., 2000, 2001, 2002, 2003]. Milan and colleagues demonstrated that the HF backscatter echoes were distributed in adjacent to discrete auroral arcs rather than being co located, which is consistent with the earlier finding with VHF radars. More recently, Bahcivan et al. [2005, 2006] used an imaging HF radar (30 MHz) to obtain detailed composite images showing the spatial relationship between auroral forms and HF backscatter. They showed that the radar aurora was often observed at the discrete arc boundary and suppressed inside the arc. On the poleward side of the latitudinally aligned auroral arcs the radar observed a narrow region of discrete backscatter echoes parallel to the arcs, whereas the echoes on the equatorward side of the arcs were more spread. They also pointed out that the radar aurora appeared to be more associated with discrete optical aurora. In particular, they showed that the radar backscatter ceased when the auroral emissions appeared to be diffuse. They suggested that the disappearance of radar aurora in the diffuse auroral feature may be attributed to the absence of strong diffuse auroral electric fields exceeding the twostream instability threshold or the absence of strong density gradients in diffuse aurora. [5] In contrast to the studies of radar aurora associated with discrete aurora, only a few studies have been performed to investigate the plasma irregularities in the vicinity of diffuse aurora. The optical definition of diffuse aurora was first introduced by Lui and Anger [1973]. They defined it as a belt of fairly uniform luminosity of low intensity that was located equatorward of auroral oval of discrete forms. Since then many satellite and ground based measurements have given a description that the diffuse aurora is spatially uniform. However, Lui and colleagues also pointed out that there exist eastward drifting auroral patches embedded within the region of diffuse aurora [Lui et al., 1973]. In addition, modern sensitive high resolution optical measurements from the ground showed that diffuse aurora sometimes consist of small scale patches of irregular shape [Sergienko et al., 2008; Shiokawa et al., 2010]. Such auroral patches are observed in the equatorward part of the auroral oval during substorm recovery phase. Sometimes, these auroral patches pulsate with a period ranging from a few seconds to a few tens of seconds, which is known as pulsating aurora [e.g., Sato et al., 2004]. Very recently, Milan et al. [2008] and Hosokawa et al. [2008] carried out detailed analyses of the characteristics of the HF radar backscatter from the E region during periods of pulsating aurora by using SuperDARN. They mainly focused on the characteristics of the Doppler parameters (i.e., Doppler velocity and spectral width) in the region of pulsating patches. However, they did not examine the spatial relationship between the pulsating auroral patches and radar backscatter in greater detail. This was simply because the pulsating period of the optical patches was too short to be tracked by using the SuperDARN HF radar. [6] In this study, we conducted a direct comparison of the auroral patches observed with a ground based all sky camera and the plasma irregularities detected by a coherent HF radar of the SuperDARN network. The fields of view (FOVs) of these two instruments have a common volume, which enables us to clarify the spatial relationship between the auroral patches and decameter scale plasma irregularities. It should be noted that the auroral patches presented in this paper were not pulsating at least during the interval studied. Thus, we simply term this type of optical form embedded within diffuse aurora region as auroral patches. As mentioned above, in the previous studies of radio and optical observations of auroral features [e.g., Bahcivan et al., 2006], spatial collocation between discrete auroral forms and associated radar aurora was investigated in detail. The major contribution of this paper is to reveal the spatial relationship of patch type aurora and its counterpart in the radar observations (i.e., plasma irregularities). 2. Experimental Arrangement [7] This paper presents the simultaneous observations of auroral patches that appeared in the postmidnight local time sector by a white light all sky CCD camera (ASC) at Tjörnes and the SuperDARN coherent HF radar at þykkvibaer in Iceland. This section contains a brief description of the two observation systems and the data sets therefrom. [8] The white light all sky CCD camera (body: WATEC WAT 120N+, lens: FUJINON YV A SA2) has been operating continuously since September 2009 at Tjörnes, which is located near the northern coast of Iceland (66.20 N, W). Auroral images are taken with an exposure time of 1.07 s (32 video frames). Figure 1a shows an example of an all sky image obtained during the interval of the present study (at 01:29:00 UT on September 18, 2009); the concentric dashed circles represent the 15, 30, 45, 60, and 75 zenith angle locii. The all sky image is aligned so that the north and east are directed toward the top and right, respectively. The exact directions of the geographic north (GN) and magnetic north (MN) are indicated at the top of this panel. This image was taken when auroral patches were passing over Tjörnes. At this time, three patches were present as diffuse and cloudy structures aligned roughly with the meridional direction. These patches are given the identifying letters A to C. [9] In this study, we assumed that the altitude of the main optical emission was 110 km. In addition to this height assumption, a comparison of the all sky images with star maps allowed us to determine the alignment of the camera accurately. The FOV of the ASC is mapped on the geographic coordinate system in Figure 1b accordingly, where the concentric blue circles indicate the contours of the 30, 60, and 75 zenith angle locii. The statistical location of the auroral oval [Feldsten and Starkov, 1967] for the prevailing geomagnetic conditions (K p = 2) is superimposed for reference. 2of10

3 Figure 1. (a) An example of an all sky auroral image obtained at Tjörnes, Iceland during the interval of the present study (at 01:29:00 UT on September 18, 2009), in which the concentric dashed circles mark the 15, 30, 45, 60, and 75 zenith angle locii. The all sky image is aligned so that the north and east are directed toward the top and right, respectively. This image was taken when auroral patches appeared over Tjörnes. Three auroral patches are given the identifying letters A to C. (b) A map showing the FOV of the SuperDARN radar at þykkvibaer and all sky CCD camera at Tjörnes, Iceland. The radar FOV is mapped assuming a backscatter altitude of 110 km. The projection of the ASC FOV is shown in blue, with three concentric circles marking the locii of the 30, 60, and 75 zenith angles mapped to an altitude of 110 km. The statistical location of the auroral oval for the prevailing geomagnetic conditions (K p =2)is superimposed. (c) Same image as Figure 1a but with the FOV of the þykkvibaer radar superimposed on the all sky image by assuming a backscatter altitude of 110 km. [10] The radar measurements were made by the coherent HF radar at þykkvibaer (63.77 N, W), which forms part of the international Super Dual Auroral Radar Network (SuperDARN) [Greenwald et al., 1995; Chisham et al., 2007]. The radar sounds along 16 different beam directions separated by 3.24 in azimuth, with the radar boresite pointing at an azimuth of 30 ( 45 ) clockwise from geographic (geomagnetic) north. Normally, radar backscatter echoes are gated into 75 range bins, usually of 45 km in length, with a range of 180 km to the first gate, giving a maximum range of 3555 km. During the campaign observation period, however, the observing mode was optimized to compare the radar data with the optical data by reducing the gate length to 15 km (the range to the first gate was also reduced to 15 km) and concentrating the soundings on ranges close to the radar. This myopic configuration increased the spatial resolution in the closer portion of the radar FOV (see Milan and Lester [2001] for details). We also changed the number of range gates from 75 to 240, which allowed us to observe the backscatter echoes from the E and F regions of the ionosphere simultaneously. [11] In addition to improving the spatial resolution with the myopic mode, we also changed the beam steering sequence and concentrated our observations on the region close to the zenith of Tjörnes. Six beams (beams 5 10) were scanned clockwise like 5, 7, 6, 7, 8, 7, 9, 7, 10, 7, with a dwell time of 2 s for each. Thus, a full scan of the FOV was completed every 20 s. At the same time, we obtained hightime resolution data along beam 7, which covered the zenith of the ASC at Tjörnes, with a temporal resolution of 4 s. This radar sounding mode allowed us to detect rapid variations in the ionospheric electric field over Tjörnes without losing the two dimensional distribution of the radar backscatter in the surrounding region. The FOV of this radar sounding mode is displayed in Figures 1b and 1c. Here, the ground range is determined from the slant range of each cell by assuming straight line propagation to an altitude of 110 km and the radar gates whose slant range is greater than 180 km are shown. It is well demonstrated in Figure 1c that the whole extent of the auroral patch (patch B in this case) was located within the radar FOV. This enabled us to examine the spatial relationship between these patch type optical forms and the associated radar backscatters. The radar operated at a frequency of 10.5 MHz, which corresponds to a wavelength of 28.6 m; thus the Bragg scatter was observed from irregularities with a wavelength of 15 m. In each radar cell, a routine analysis of the obtained auto correlation functions provides the backscatter power, spectral width, and Doppler shift of the echo spectra [Baker et al., 1988]. 3. Observations [12] The interval presented in this paper is UT on September 18, Figure 2a shows the keogram reproduced from the Tjörnes ASC images along the West to East cross section, in which the optical intensity is displayed with white indicating the brightest pixels and black representing the darkest pixels. Note that the optical data before 0120 UT cannot be used for the scientific investigation because the weather was slightly cloudy. In the keogram, slanted traces of bright optical intensity drifting from west to east are clearly seen throughout the interval, which are a manifestation of eastward drifting auroral patches. In particular, the three individual patches present in the all sky image of Figure 1a (patches A, B, and C) are 3of10

4 Figure 2. (a) Keogram of Tjörnes ASC along the west to east cross section for the interval of 0100 to 0200 UT on September 18, (b d) RTI plots of the backscatter power, Doppler velocity, and Doppler spectral width from beam 7 of the þykkvibaer radar. The vertical axis is the range gate, with a 15 km separation between consecutive gates. (e) H component magnetic field variation at Tjörnes. apparent in the middle of this interval. The eastward motion of the patches corresponds to the sunward plasma drift in the postmidnight westward electrojet, suggesting that the auroral patches were drifting with the background plasma convection. The speed of this eastward motion, as estimated from the traces in the keogram, varied to some extent between the three patches. However, it ranged from 360 to 450 m s 1, which will be compared with the Doppler velocity observations from SuperDARN in a later part of this section. [13] Figures 2b 2d respectively show the Range Time Intensity (RTI) plots of the backscatter power, line of sight Doppler velocity, and spectral width observed along beam 7 of the SuperDARN þykkvibaer radar, in which the zenith of Tjörnes is indicated with the horizontal dashed line. The 4of10

5 temporal resolution of the radar data is 4 s. As the auroral patches passed through the zenith of Tjörnes, the þykkvibaer radar observed some blobs of strong radar backscatter in range gates 17 30, suggesting that the optical patches had a close association with the radar backscatter echoes (i.e., plasma irregularities). We identified the current radar echoes as originating from the E region because the radar range in which they appeared was very close to the radar site. In the previous work of irregularities using SuperDARN, the echoes appeared in the range closer to 600 km have been regarded as E region irregularities [Ruohoniemi and Greenwald, 1997]. The echoes we describe in this paper appear at a slant range of km, which well meets the criteria of E region irregularities in the SuperDARN measurements. We will discuss the spatial relationship between the auroral patches and the radar backscatter echoes in greater detail in a later part of this section by showing two dimensional maps of the optical and radar data. [14] The interpretation of the line of sight Doppler velocities (Figure 2c) is somewhat difficult since beam 7 of the þykkvibaer radar is directed northeastward (45 clockwise from the magnetic north). However, the Doppler velocities within the blobs of irregularities are generally negative (away from the radar) over Tjörnes in most of the interval presented, which is consistent with the eastward motion of the auroral patches in the optical data. The magnitude of the Doppler velocity varied in time. However, it varied in time from approximately 100 to 300 m s 1 during the passage of patches A, B, and C. If we assume that these Doppler velocities are the line of sight component of the L shell aligned eastward plasma convection, these values can be converted to 150 to 450 m s 1 since beam 7 of the þykkvibaer radar is directed northeastward (45 clockwise from the magnetic north). These estimated background plasma drifts are nearly consistent with the speed of the eastward motion of the auroral patches as derived from the optical observations, although there existed an occasion in which the radar Doppler velocity was lower (around 100 m s 1 ) than the line of sight component of the moving speed of the patches. This overall correspondence between the radar velocity and speed of the patches again suggests that the auroral patches drifted with the background plasma convection. [15] The spectral width within the blobs was extremely narrow, less than 100 m s 1 most of the time. Hosokawa et al. [2008] reported that the spectral width within a patch of pulsating aurora was also extremely narrow as compared with that in some discrete aurora. Thus, the narrow spectral width observed during the present interval may indicate that the Doppler spectra of the coherent HF radar backscatter from auroral patches within diffuse aurora are generally narrow. Figure 2e shows the ground magnetic field H component obtained from a fluxgate magnetometer at Tjörnes. The ground magnetic field was directed predominantly southward. This indicates that the all sky camera and the radar sensing area were located within a channel of westward electrojet in the postmidnight local time sector, which is consistent with the eastward Doppler velocities obtained by the þykkvibaer radar. [16] In order to compare the two dimensional spatial distributions of the auroral patches and radar backscatters in a more direct way, we overplot the radar data on the all sky optical images in Figure 3. The optical images in the panels on the left hand side were taken at 1 min intervals between 0127 UT and 0132 UT. The backscatter power and Doppler velocity obtained from the radar are superimposed on the all sky images in the panels in the center and right hand side, respectively. During the interval presented in Figure 3, patch A almost moved out of the FOV, patch B was passing through the zenith of Tjörnes, and patch C was approaching the zenith of Tjörnes from the western part of the FOV. Let us first discuss the spatial distribution of the backscatter echoes shown in the center panels of Figure 3. At 0127 UT, the radar echoes were distributed in the dark region sandwiched by patches A and B. As patches A and B drifted eastward, this echo region moved in the same direction. At 0129 UT, a new radar echo region appeared on the western part of the radar FOV, which was sandwiched by patches B and C. At 0132 UT, this new echo region reached the zenith of Tjörnes, and it is clearly seen that the radar echoes were observed only within the gap between patches B and C. A direct comparison of the two dimensional spatial distributions of the optical patches and radar echoes demonstrates that the decameter scale plasma irregularities were distributed in the gap between the auroral patches. An animation showing the evolution of the optical patches and radar backscatter echoes at a rate of one frame every 20 s accompanies the electronic version of this article. Movie S1 shows that radar echoes were found to be distributed in the gaps between the auroral patches and to move eastward almost in tandem with the auroral patches. 1 [17] In order to confirm the spatial collocation of the optical patches and radar echoes shown in Figure 3 (i.e., that the radar echoes were distributed in the gaps between the auroral patches), we created an RTI plot of the optical data, which is shown in Figure 4c. To create this plot, we selected the optical data at each radar range gate along beam 7 of the þykkvibaer radar and plotted it in the same format as the radar RTI plots shown in the top two panels of Figure 4. In Figure 4c, patches A, B, and C are seen as slanted traces drifting away from the radar. We traced the regions of higher backscatter power (>15 db) in Figure 4a and overlaid them on Figure 4c as blue contours. Regions of high backscatter power are clearly found to exist in the gaps between the auroral patches. This again confirms that the radar echoes favor the dark areas sandwiched by the auroral patches. [18] The contours of the high backscatter regions are also superimposed on the RTI plot of the Doppler velocity (Figure 4b) with blue lines. During the interval presented in Figure 3 ( UT), radar backscatter was absent in the regions of auroral patches. However, as shown in Figure 4, there were radar backscatter echoes wrapped around patch A (between 0122 to 0127 UT), although the obtained backscatter power was considerably smaller than that of the echoes in the gap region. An important feature to note is that the Doppler velocities from this low backscatter power region were clearly less than those from the surrounding regions of higher backscatter power. This indicates that the electric field was significantly reduced in the region of a auroral patch, which might have contributed to the reduction of plasma irregularities in this region. In the next section, we will discuss the absence of radar backscatter in the regions 1 Auxiliary materials are available in the HTML. doi: / 2010JA of10

6 Figure 3 6 of 10

7 Figure 4. (a) RTI plot of the backscatter power along beam 7 from 0120 to 0140 UT on September 18, (b) RTI plot of the Doppler velocity along beam 7, with the regions of high backscatter power (>15 db) outlined with blue contours. (c) Optical data in the RTI format, with the regions of high radar backscatter power superimposed with blue contours. of auroral patches in terms of the electric field reduction associated with auroral particle precipitations. 4. Discussion [19] As demonstrated in the previous section, strong radar backscatters were detected in the gaps between the auroral patches, while echoes were absent or very weak within the patches. Another interesting point is that the Doppler velocities within the patches were found to be smaller than those in the gap region. This suggests that the electric field was reduced locally within the patches, possibly in association with an enhancement of the ionospheric conductance due to auroral particle precipitation. Such a reduction in the electric field could be a key to account for the absence of the radar backscatter within the auroral patches. [20] Ionospheric plasma irregularities are density fluctuations that have been amplified by plasma instability processes such as the gradient drift instability and/or two stream instability [Fejer and Kelley, 1980; Keskinen and Ossakow, 1983; Tsunoda, 1988]. If we assume that the radar echoes observed during the current interval were generated by the gradient drift instability, the linear growth rate of the density fluctuation, g, is proportional to the background plasma velocity, V 0 (this corresponds to the E B drift velocity of electrons at E region altitudes because the ion population is constrained to the neutral flow speed due to the high ionneutral collision frequency), and inversely proportional to the scale length of the background plasma density gradient, L, i.e., g / V 0 / L, where V 0 = E 0 / B, L = n 0 / r n 0. Here, B is the magnitude of the geomagnetic field, E 0 is the magnitude of the electric field in the plane perpendicular to B, and n 0 is the background plasma density. [21] We consider that the echoes adjacent to the auroral patches were produced primarily through the gradient drift instability in the E region because the measured Doppler velocities were slightly below the threshold of an excitation of the two stream instability. However, due to the turbulent nature of the instability process, the mean Doppler velocity along any particular viewing direction does not need to exceed the ion acoustic velocity to generate Type I echoes. If the velocity along the electron drift direction is greater than the ion acoustic velocity, Type I irregularities can be generated. The Doppler velocities obtained in the dark regions between the auroral patches were not exceeding but very close to the threshold of Type I echoes; thus a hybrid form of two stream and gradient drift instability may be occurring. [22] In the E region, the gradient drift instability works more efficiently when the background electric field and the density gradient are in the same direction. During the interval of interest, the background electric field was directed southward because the auroral patches were drifting eastward. Thus, we need a southward density gradient to operate the gradient drift instability. We suppose that there existed a large scale equatorward gradient in the electron density in this latitudinal region. Such an electron density gradient could cause the radar echoes in combination with the southward directed background electric field. [23] Although generation of irregularities through the gradient drift instability also depends on the scale length and direction of the background electron density gradient, the irregularities primarily favor regions with a strong electric field. In reality, Milan et al. [1999] demonstrated an absence of gradient drift instability related coherent radar backscatter in low electric field regions. During the interval of interest, radar echoes were found to be absent or very weak within Figure 3. (left) All sky images at 1 min intervals from 0127 to 0132 UT on September 18, (middle) All sky images from the same period, with the backscatter power from the þykkvibaer radar superimposed. (right) The same images with the Doppler velocity data overplotted. 7of10

8 and 0140 UT as time series, respectively. All of the data shown here were sampled at range gate 23 along beam 7, which roughly corresponded to the zenith of Tjörnes. The three outstanding peaks in the optical data (Figure 6a) cor- Figure 5. A scenario for the reduction in the equatorward electric field within an auroral patch. See text for details. the auroral patches where the electric field was significantly reduced. This situation is fairly consistent with the generation of irregularities by the gradient drift instability. That is, radar echoes could not survive within the auroral patches because the electric field was not sufficient for maintaining the gradient drift waves. [24] Now we turn to a discussion of the details of the process through which the electric field was reduced within the auroral patches. We intend to interpret this electric field reduction as a consequence of an enhancement of the Hall conductance within the auroral patch. The ionospheric Hall conductance should be significantly enhanced in the region of the patches by auroral particle precipitation. In order to keep the westward flowing Hall current continuous, the electric field must be reduced in the region of enhanced conductance (i.e., within the auroral patches). Figure 5 shows a schematic illustration of this scenario. In Figure 5, the three auroral patches are drifting eastward with a background eastward convection (V 0 ) of approximately 400 m s 1. This eastward plasma drift is identical to an electric field directed equatorward (E 0 ). If we take B to have a value of nt (IGRF) over Iceland, the eastward drift of 400 m s 1 is equivalent to an equatorward electric field of 20 mv m 1 ; this will be used as the value of the background electric field in the following discussion. In the westward electrojet, the equatorward electric field generates a Hall current (J H ) flowing westward through the auroral patches, which can be expressed as J H = S H0 E 0, where S H0 is the height integrated Hall conductance outside the auroral patches. Within the auroral patches, the Hall conductance is enhanced by the particle precipitation (shown as gray ellipses in Figure 5). Here, we assume that the spatial structure of the auroral patches is elongated in the meridional direction, which is based on the all sky images shown in Figure 1a. In order to keep the westward flowing Hall current continuous, the electric field in the region of enhanced conductance must be reduced (i.e., E 0 is reduced to E ). This reduction in the electric field is then responsible for the absence of radar echoes within the auroral patches. [25] In order to test this scenario, we tried to reproduce the line of sight Doppler velocities by assuming the conductance distribution in the vicinity of the patches and then compared the reproduced velocities with those observed by the radar. Figures 6a, 6f, and 6g show the optical intensity, Doppler velocity, and backscatter power between 0120 UT Figure 6. (a) Temporal variation in the white light optical intensity at gate 23 along beam 7. The gaps between the auroral patches are highlighted in gray to aid discussion. (b) The height integrated Hall conductance estimated from the white light optical intensity. (c) The magnitude of the westward flowing Hall current. (d) Reproduced equatorward electric field. (e) Reproduced line of sight Doppler velocity. (f and g) The Doppler velocity and backscatter power obtained from the þykkvibaer radar at gate 23 along beam 7. 8of10

9 respond to the passages of patches A, B, and C. In Figure 6, the gaps between the auroral patches are hatched with gray to demonstrate how the radar data varied as the patches traversed the radar sensing area. As described in the previous section, no radar echoes were observed or they were significantly weak within the auroral patches (Figure 6g). In addition, the Doppler velocities (Figure 6f) within the auroral patches were clearly smaller (<200 m s 1 ) than those in the gap region (up to 300 m s 1 ), which is more clearly seen during the passage of patch A. [26] To reproduce the reduction in the electric field within the auroral patches, we first need to determine the spatial distribution of the Hall conductance in the vicinity of the patches. Here, we assume that the Hall conductance is proportional to the optical intensity of the white light, as S H = 0.35 I opt, where S H is the height integrated Hall conductance in mho and I opt is the optical intensity of the white light, as shown in Figure 6a. Figure 6b shows the height integrated Hall conductance derived in this way. The Hall conductance was elevated from 5 to 8 mho to mho when the auroral patches appeared at the zenith of Tjörnes. Recently, Hosokawa et al. [2010] used the EISCAT radar to measure variations in the ionospheric conductance associated with the occurrence of pulsating aurora. They demonstrated that the Hall conductance was elevated by a factor of 2 or more when patches of pulsating aurora appeared at the sensing area of EISCAT, which suggests that the Hall conductance enhancement assumed here is reasonable. In addition, we employed 5 mho as the background value for the Hall conductance outside the auroral patches (S H0 ) to estimate the magnitude of the Hall current. [27] By using the background values for the electric field ( E 0 =20mVm 1 ) and Hall conductance (S H0 = 5 mho), the magnitude of the Hall current flowing westward (J H = S H0 E 0 ) could be estimated to be 0.1 A m 1, which is shown in Figure 6c. By dividing J H by S H, the distribution of the electric field can be modeled as shown in Figure 6d. It can be seen that the reduction in the electric field within the auroral patches is reproduced well. Namely, it is clear that there is less of an electric field within the patches (<10 mv m 1 ) than in the gap region (10 25 mv m 1 ). Figure 6e shows the reproduced line of sight Doppler velocities. The overall agreement between the reproduced (Figure 6e) and observed (Figure 6f) Doppler velocities was satisfactory. In particular, the temporal variation in the Doppler velocity data from the radar measurement was well reproduced by the quantitative estimation, which confirmed the validity of the scenario illustrated in Figure 5. However, the Doppler velocities reproduced from the current/conductance analysis are somewhat underestimated especially both at the trailing edge of patch A and in the gap between patches A and B. This is probably because the estimation of the conductance from the white light optical intensity is not correct at these times. [28] In the past, only a few studies have focused on the current and electric field structure in the vicinity of longitudinally aligned auroral forms. Opgenoorth et al. [1983] used the STARE VHF radar to observe the two dimensional electric field structure in a sequence of auroral omega bands drifting eastward. Auroral omega bands are large scale undulations in the poleward boundary of auroral oval in the morning sector; thus, they are quite different from the auroral patches in the current observations. However, they demonstrated that the spatial structures of the current and electric field were modified by an inhomogeneous ionospheric conductivity associated with the passage of longitudinally aligned auroral torches in the omega bands. The similarity between their results and current observations may support our model of electric field reduction in the region of enhanced conductance associated with the auroral patches. [29] In summary, we report a close association between decameter scale plasma irregularities in the E region ionosphere and auroral patches. The simultaneous radar and optical measurements demonstrated that there were strong radar backscatter echoes within the gaps between adjacent auroral patches, while no echoes were observed or were very weak in the regions of the auroral patches. The Doppler velocity estimation showed that the electric field was clearly reduced in a region of auroral patch, which was probably the result of the enhanced conductance associated with auroral precipitation. This reduction in the electric field, then, suppressed the generation of irregularities (i.e., radar aurora) in the regions of auroral patches. This suggests that the conductance enhancement associated with precipitating electrons not only modified the electric field within the aurora but also affected the generation of small scale plasma structures in the vicinity of patch type optical auroral forms. It is well established that HF backscatter is adjacent to and between auroral arcs [e.g., Milan et al., 2002, Bahcivan et al., 2006]. 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