First radar observations in the vicinity of the plasmapause of pulsed ionospheric flows generated by bursty bulk flows



[1] Recent expansion of the SuperDARN network to mid-latitudes and the addition of a new high-time resolution mode provides new opportunities to observe mid-latitude ultra-low frequency waves and other ionospheric sub-auroral features at high temporal resolution. On 22 February 2008, the Blackstone SuperDARN radar and THEMIS ground magnetometers simultaneously observed substorm Pi2 pulsations. Similarities in measurements from the Blackstone radar and a magnetometer at Remus suggest a common generating mechanism. Cross-phase analysis of magnetometer data places these measurements at the ionospheric projection of the plasmapause, while fine spatial and temporal details of the radar data show evidence of field line compressions. About 1 min prior to ground Pi2 observation, 2 Earthward-moving Bursty Bulk Flows (BBFs) were observed by THEMIS probes D and E in the near-Earth plasma sheet. We conclude that the first 2 pulses of the Pi2s observed at Blackstone and Remus result from compressional energy generated by BBFs braking against the magnetospheric dipolar region.

1. Introduction

[2] Pi2 refers to a class of ultra-low frequency (ULF) geomagnetic pulsations with an irregular waveform and a period of 40–150 s [Jacobs et al., 1964]. They are thought to be generated by processes driven by current-carrying Alfvén waves or compressional waves produced during substorm onset [Olson, 1999]. Alfvén waves, which can be generated by a diversion of the cross-tail current during substorm current wedge (SCW) formation [Baumjohann and Glassmeier, 1984], are associated with high-latitude Pi2s. Compressional energy is thought to drive lower-latitude Pi2s, but Olson [1999] notes that the generating mechanism of compressional waves at substorm onset is unknown. Fujita et al. [2002] use numerical modeling to show how SCW formation can generate a compressional impulse; Kepko and Kivelson [1999] and Kepko et al. [2001] suggest that such energy could be produced by the braking of Earthward-moving bursty bulk flows (BBFs) against the magnetospheric dipolar region due to a pressure gradient. BBFs are high speed, convective plasma flow bursts found in the inner plasma sheet thought to be caused by transient reconnection in the magnetotail [Sergeev et al., 1992; Angelopoulos et al., 1994]. Two mechanisms have been suggested for Pi2 generation from this compressional energy. The cavity mode resonance model (CMR) suggests that standing waves are excited in the plasmaspheric cavity [Takahashi et al., 2001; Nosé, 2010], while the direct response BBF model (DR-BBF) suggests ground instruments directly sense compressional waves generated by BBF braking [Kepko and Kivelson, 1999; Kepko et al., 2001]. In CMR, Pi2 period depends on plasmaspheric parameters; in DR-BBF, it depends on the inter-BBF period. Kepko and Kivelson [1999] and Kepko et al. [2001] found events which support DR-BBF; however, the studies do not show this conclusively. We present a case study using radar, ground magnetometer (GMAG), and satellite data which gives clear evidence of a ground Pi2 resulting from compressional energy generated by BBFs braking against the magnetospheric dipolar region.

[3] The Blackstone radar (BKS) (37.10°N, 77.95°W) began operation in February 2008 as part of an effort to expand SuperDARN [Chisham et al., 2009, and references therein] radar capabilities to the mid-latitude region. SuperDARN range resolution is 45 km, which corresponds to a 0.3° latitudinal resolution around 55°N magnetic latitude. The radar measures the line-of-sight (LOS) component of the E × B drift velocity of F region plasma when decameter-scale ionization irregularities are present and so can detect the electric field component of ionospheric pulsations. Previous studies of mid-latitude pulsations with SuperDARN radar techniques include those of Gjerloev et al. [2007] and Greenwald et al. [2008].

[4] The NASA THEMIS mission consists of ground based observatories (GBOs) [Mende et al., 2008] spread across North America and five identical satellites in highly elliptical orbits [Angelopoulos, 2008]. Each GBO consists of an all-sky imager and a fluxgate GMAG capable of detecting perturbations in the magnetic field that represent the integrated effects of ionospheric currents flowing on spatial scales of hundreds of kilometers.

2. Observations

[5] We present an isolated substorm after a period of extended geomagnetic quiet conditions. On 22 February 2008 at 0436 UT, the auroral electrojet (AE) index rose from 25 to 175 nT over a 25 min period, and had been steady at 25 nT for 2 hr prior. The solar wind interplanetary magnetic field (SW IMF) Bz at the nose of the Earth's bow shock from the OMNI database [King and Papitashvili, 2006] turned southward at 0415 UT and varied between −2 and +1 nT until 0500 UT. During this interval the SW velocity vx was ∼490 km s−1 and the proton density was ∼3.0 cm−3. Liu et al. [2009] conducted a study of associated high-latitude phenomena.

[6] Figure 1 is an AACGM [Baker and Wing, 1989] map of the 22 February event. Brown asterisks indicate the locations of THEMIS GBOs. At 0436 UT, the Eastern portion of the GBO network was centered around midnight magnetic local time (MLT), allowing for good coverage of substorm activity. Beginning at 0436 UT, Pi2s were observed by GMAGs at all GBOs shown in Figure 1. Figure 2e shows the magnetogram from RMUS. Pi2 signatures (not shown) from other midlatitude THEMIS GBOs are similar, although a longitudinally dependent phase shift in nightside Pi2s can be observed. Pi2s were also seen by southern hemisphere magnetometers in the SAMBA (South American B-Field Array) chain and dayside magnetometers in the STEP 210 chain [Yumoto and C. P. M. N. Group, 2001]. Bx component magnetograms (not shown) from STEP 210 stations at Rikubetsu (37.29°N, 144.71°W AACGM, 1351 MLT) and Kototabang (16.32°N, 131.82°W AACGM, 1443 MLT) reveal Pi2s that are simultaneous and in-phase with RMUS Bx, although one-tenth the amplitude. Optical observations were limited due to generally cloudy conditions. However, white-light flux from Gillam (GILL) shows a sharp increase starting at 0436 UT [Liu et al., 2009]. Figure 1 also shows the magnetic footpoints of THEMIS D and E (THM-D,E) as determined by the T96 magnetic field model [Tsyganenko, 1996]. At 0436 UT, these probes were located at (−10.9,3.3, −2.3) and (−10.2,4.1, −2.1) RE GSM, respectively, and were within 0.20 RE of the neutral sheet.

Figure 1.

AACGM map for the substorm on 22 February 2008 at 0436 UT. The map includes LOS BKS velocity data at 0444-0446 UT (green-red-blue colors), POES auroral precipitation flux at 0430-0530 UT (blue colors), locations of GBOs (brown asterisks), footprints of two THEMIS probes (letters D and E), estimates of the auroral oval (grey circles), westward electrojet (pink color), SCW FACs (encircled dots), and plasmapause (between blue dashed lines).

Figure 2.

Time series data for a substorm on 22 February 2008. (a) BKS radar LOS velocity data from beam 8 with 2 min resolution. (b) BKS beam 7 shows Pi2s which cannot be observed in beam 8. (c) LOS velocity data from BKS beam 7, range gate 21. (d) RMUS GMAG component rotated into the LOS direction of the radar, baseline removed. (e) 3-component RMUS data, baseline removed. (f) Ion velocity data from THM-D and E.

[7] An estimate of the auroral oval location for Kp = 1 [Holzworth and Meng, 1975] is shown in Figure 1. Measurements from the Total Energy Detector (TED) instrument on the NOAA POES satellite [Evans and Greer, 2006] have been overlaid and provide reasonable agreement. Also shown are the longitudinal estimates of the upward and downward field aligned currents (FACs) [Liu et al., 2009], as well as the position of the westward electrojet. Using a cross-phase analysis technique developed by Waters et al. [1991], we estimate the ionospheric projection of the plasmapause to be located between L = 3 − 3.71 (54°–58° AACGM latitude), as indicated by the region between the blue dashed lines near the bottom of the map.

[8] BKS LOS velocity measurements are shown in Figure 1 between 2200 and 0000 MLT and between 50° to 60° AACGM latitude. These velocities correspond to a 2 min scan over 16 beams beginning at 0444 UT. The thick dashed line indicates the location of the THEMIS mode “camping” beam, which provides 6 s resolution by interleaving camping beam measurements between those of normal beams. It can be seen that the BKS measurements are located at the ionospheric projection of the plasmapause in the pre-midnight subauroral region within a longitudinal sector defined by the SCW FACs.

[9] Figure 2 presents time series data from the BKS radar, the RMUS GMAG, and THM-D and E. Both Figures 2a and 2b show range-time plots of BKS plasma velocity data from 54° to 58° AACGM latitude. Figure 2b shows data from the 6 s resolution camping beam while Figure 2a is from an adjacent 2 min resolution beam. The camping beam data of Figure 2b reveals fine structure in time and space that cannot be seen in Figure 2a, including ULF velocity pulsations with a latitudinal saw-tooth structure.

[10] Figures 2c and 2d compare measurements from the BKS camping beam at range gate 21 (54.79°N, 12.49°W AACGM) with data from RMUS located at (54.65°N, 12.64°W AACGM). The data from RMUS have been average subtracted and rotated into the radar LOS look direction. Figure 2e shows the 3-component RMUS data for comparison. Many similarities can be seen when comparing the BKS data to the RMUS data, most notably in the waveforms of the Pi2 pulsations which begin ∼0436 UT. The onset of the Pi2 in both the BKS and RMUS data correspond in time within one wave period to the auroral brightening at GILL and the observed AE enhancement.

[11] Differences in observations allow the data sets to complement each other. In Figure 2e, the RMUS GMAG provides information about large scale current systems and 3-dimensional measurements of the magnetic field. In Figures 2b and 2c, the radar provides localized measurements of the ionospheric electric field which contain significant spatial and temporal detail.

[12] About 1 min prior to ground observations of Pi2s, THM-D and E observed BBFs. Figure 2f shows ion velocities measured by the Electrostatic Analyzers (ESA) [McFadden et al., 2008] on both THM-D and E. Two BBFs corresponding to Earthward-moving velocity pulses can be seen in each data set.

3. Discussion

[13] The resemblance of the BBF velocity pulses to the waveforms measured by RMUS and BKS is striking. In THM-D data, the time between BBF pulse peaks is ∼135 s, while the time between the peaks of the first 2 pulses of both the RMUS and BKS data is ∼138 s. The difference between these times is less than the time resolution of BKS, and the similarities in ground and space observations are suggestive of the DR-BBF model. We note that the peak of the second THM-E BBF occurs prior to that of THM-D. However, as the velocity amplitude of the second THM-D BBF is significantly higher than that of THM-E, we believe it is likely that the compression from the second THM-D BBF will extend that of THM-E. Therefore, it is the peak of the second THM-D BBF that is important for timing.

[14] Figure 3 provides a detailed look at data first presented in Figure 2b. In Figure 3a, BKS beam 7 range gates 10–24 are averaged together in adjacent groups of 3 to create 5 velocity traces. Figure 3b is obtained by subtracting the mean background velocity from each trace. We note that Pi2s in Bx magnetograms (not shown) from THEMIS GBOs in Shawano (SWNO) (55.72°N, 17.62°W AACGM), Derby (DRBY) (54.99°N, 6.16°E AACGM), and RMUS are in phase, and thereby conclude that the phase relationships shown in Figure 3 depend only on latitude. Each trace corresponds to a different L-shell and represents the motion of different length field lines. We define coherence as identical velocities among traces after the removal of background velocities. Coherence should not be expected in nominal cases. Figure 3b reveals periods of both coherence and dispersion, as exemplified by the second pulsation. This coherence begins at 0440 UT (indicated by arrow) and continues until pulse maximum, which is marked as the beginning of the second dispersion. We interpret the coherence as a signature of compression by the second BBF, and the dispersion as a signature of system relaxation. During the relaxation, the highest latitude measurements change the slowest because they correspond to the movement of the longest field lines. From the second dispersion until the end of the presented interval, the traces exhibit little coherence as the forcing of the BBF has gone. Coherence and dispersion are also seen in the first pulse of Figure 3b; this corresponds to compression by the first BBF.

Figure 3.

(a) Three range gate averages of BKS beam 7 LOS velocity data. (b) Same as Figure 3a, but average subtracted. During intervals marked as coherences, the velocity traces from all latitudes move together as energy from BBF braking pushes on the field lines. During intervals marked as dispersions, field lines move back toward their original state.

[15] As an additional test, we estimated the signal transit time of the maxima of the first pulse from THM-D to the L-shell of the highest latitude BKS measurement. We first separated the signal path into 2 regions. In the first region, the BBF travels in a straight line at the initially measured velocity vx = 570.3 km s−1 from THM-D at −10.9 RE to a braking region at −10 RE X-GSM [Birn et al., 1999]. In the second region, we assumed energy was converted to a compressional wave traveling at the local Alfvén velocity (ranging from 270 to 3330 km s−1) along the equatorial plane until the L-shell of the outermost BKS measurement at −2.8 RE X-GSM. The equatorial plane location and the magnetic field values for the Alfvén velocities came from the T96 model using disturbance storm time index DST = 2.5 nT, SW dynamic pressure p = 1.4 nPa, IMF By = 3.5 nT GSM, and IMF Bz = −0.5 nT GSM. Particle density was determined using THM-D measurements and a R−3 dependence from the center of the Earth [Clausen et al., 2008]. This results in a total transit time of 80 s. The observed first pulsation maximum at THM-D occurred at 0437:14 UT while the corresponding first maximum at BKS occurred at 0438:32 UT, giving an observed transit time of 78 s. This agrees well with our estimate.

[16] We have shown that DR-BBF mechanisms drive the first 2 ground Pi2 pulses. It is possible these pulses provide energy that excites a CMR [Fujita et al., 2002; Zhu and Kivelson, 1989]. This could account for the Pi2's global nature, the Pi2's continued ringing after the initial 2 pulses, and the simultaneity of the noon-midnight observations [Sutcliffe and Yumoto, 1989].

4. Conclusions

[17] We have presented the first direct evidence of BBFs producing compressions at the plasmapause. During an isolated substorm on 22 February 2008 at 0436 UT, Pi2s were observed simultaneously by the BKS SuperDARN radar and THEMIS GMAGs. Just over 1 min prior to observation of ground Pi2s, 2 Earthward-moving BBFs were observed by THM-D and E in the near-Earth neutral sheet. Similarities in pulsations detected by the BKS radar and the RMUS GMAG suggest that a single source generated the Pi2s observed by these stations. Both stations were located in the pre-midnight sector of the sub-auroral region. Previously, these Pi2s could have been interpreted to be generated by the transient response SCW mechanism [Baumjohann and Glassmeier, 1984], but we find that the first 2 Pi2 pulses resulted from DR-BBF mechanisms. These compressions may also couple to a CMR. This conclusion is based on similarities of the BBF waveforms to the first 2 pulses observed by BKS and RMUS, a propagation timing analysis, and the prior work of Kepko and Kivelson [1999], and Kepko et al. [2001]. Additional evidence is derived from further study of the BKS radar data, which shows a coherence in the velocity perturbation with latitude as the pulses rise to maximum, but a dispersion in the velocities as the system relaxes. Cross-phase analysis of GMAG data places these measurements at the ionospheric projection of the plasmapause. Therefore, we interpret these latitudinal variations in the radar data as evidence of compressions at the plasmapause.


[18] Support for this research and funding for the construction of the Blackstone SuperDARN radar is provided by NSF grants ATM-0849031 and ATM-0946900. K. Oksavik thanks the Research Council of Norway for financial support. Kp and AE indices were obtained from the WDC in Kyoto. We acknowledge NASA contract NAS5-02099 and V. Angelopoulos for use of data from the THEMIS Mission. Specifically C. W. Carlson and J. P. McFadden for use of ESA data, S. Mende, C. T. Russell, and I. Mann for use of GMAG data, and the CSA for support of the CARISMA network. We thank J. Green for providing data from the NOAA/POES TED instrument, and E. Zesta for SAMBA GMAG data. STEP 210 GMAG data was copied from the Solar-Terrestrial Environment Laboratory, Nagoya University. SW and IMF data was obtained from the CDAWeb OMNI database by J. H. King and N. Papitashvili. Satellite positions and magnetic footprints were obtained with the TIPSOD program by NASA GSFC SSC.