Periodic traveling compression regions during quiet geomagnetic conditions and their association with ground Pi2

Abstract

Abstract. Recently, Keiling et al. (2006) showed that periodic (∼90 s) traveling compression regions (TCRs) during a substorm had properties of Pi2 pulsations, prompting them to call this type of periodic TCRs "lobe Pi2". It was further shown that time-delayed ground Pi2 had the same period as the lobe Pi2 located at 16 R E , and it was concluded that both were remotely driven by periodic, pulsed reconnection in the magnetotail. In the study reported here, we give further evidence for this association by reporting additional periodic TCR events (lobe Pi2s) at 18 R E all of which occurred in succession during a geomagnetically very quiet, non-substorm period. Each quiet-time periodic TCR event occurred during an interval of small H -bay-like ground disturbance (<40 nT). Such disturbances have previously been identified as poleward boundary intensifications (PBIs). The small H bays were superposed by Pi2s. These ground Pi2s are compared to the TCRs in the tail lobe (Cluster) and both magnetic pulsations and flow variations at 9 R E inside the plasma sheet (Geotail). The main results of this study are: (1) Further evidence is given that periodic TCRs in the tail lobe at distances of 18 R E and ground Pi2 are related phenomena. In particular, it is shown that both had the same periodicity and occurred simultaneously (allowing for propagation time delays) strongly suggesting that both had the same periodic source. Since the TCRs were propagating Earthward, this source was located in the outer magnetosphere beyond 18 R E . (2) The connection of periodic TCRs and ground Pi2 also exists during very quiet geomagnetic conditions with PBIs present in addition to the previous result (Keiling et al., 2006) which showed this connection during substorms. (3) Combining (1) and (2), we conclude that the frequency of Correspondence to: A. Keiling (keiling@ssl.berkeley.edu) PBI-associated Pi2 is controlled in the outer magnetosphere as opposed to the inner magnetosphere. We propose that this mechanism is pulsed reconnection based on previous results which combined modeled results and observations of substorm-related periodic TCRs and ground Pi2. (4) We show that TCRs with small compression ratios ( B/B<1%) can be useful in the study of magnetotail dynamics and we argue that other compressional fluctuations with B/B<1% (without having all of the characteristic signatures of TCRs) seen in the tail lobe could possibly be related to the same mechanism that generates TCR with B/B>1% (which are more commonly studied). (5) Finally, it is noted that both quiet time and substorm-related periodic TCRs had remarkably similar periods in spite of the drastically different geomagnetic conditions prevailing during the events which poses the important question of what causes this periodicity under these different conditions.

The dependence of Pi2 waveforms on periodic velocity enhancements within bursty bulk flows

“…That is, the in-situ magnetic signature of the Pi2 was observed ∼30 s prior to that visually identified in the magnetometer time-series on the ground. Further, Keiling et al (2008) demonstrated that there was, at times, a magnetic perturbation signature with an amplitude of ∼0.5 nT observed by Geotail at 8-9 R E . Taken together, these results were used to propose that a common source for ground-based and insitu Pi2s, specifically transient reconnection, which could account for both the short time delay between the signals as well as the similar frequency content of the Pi2 waveforms.…”

“…In their studies, Keiling et al (2006Keiling et al ( , 2008 found that there was a small time delay observed between the magnetic signature observed at Cluster situated in the nightside magnetosphere (16-18 R E ) and mid-auroral latitude magnetometers (L = 4.5-6.1). That is, the in-situ magnetic signature of the Pi2 was observed ∼30 s prior to that visually identified in the magnetometer time-series on the ground.…”

“…A common source in the magnetotail could, for instance, be bursty/non-steady reconnection (Keiling et al, 2006(Keiling et al, , 2008 In such a scenario, bursty reconnection, potentially driven by an irregular inflow of ions into the NENL reconnection region, could produce both a propagating compressional disturbance as well as a bulk plasma flow burst with similar temporal structure. Each subsequent "burst" of reconnection could in turn generate an additional compressional disturbance and plasma flow.…”

“…Interestingly, Keiling et al (2006Keiling et al ( , 2008 presented case studies during differing nightside magnetotail driving conditions whereby favourable conjunctions between in-situ spacecraft between 8-18 R E were utilised to study the ground-based and in-situ signatures of Pi2s. In their studies, Keiling et al (2006Keiling et al ( , 2008 found that there was a small time delay observed between the magnetic signature observed at Cluster situated in the nightside magnetosphere (16-18 R E ) and mid-auroral latitude magnetometers (L = 4.5-6.1).…”

“…Tamao, 1964). Finally, Kepko and Kivelson (1999) suggested that Pi2s may be directly-driven by the deceleration of large amplitude earthward plasma flows, whilst Keiling et al (2006Keiling et al ( , 2008 proposed that high-latitude Pi2 signals may directly represent the signatures of transient magnetotail reconnection, as well as a number of other Pi2 waveforms observed on the ground. Resolving the generation mechanism, or mechanisms, responsible for the generation of ground Pi2 signals is therefore of importance to fully understanding the magnetotail drivers of ground-based signatures.…”