We statistically study the local time distribution of the helium band electromagnetic ion cyclotron (EMIC) waves observed at geosynchronous orbit when geomagnetic activity was low (Kp ≤ 1). In order to identify the geosynchronous EMIC waves, we use high time resolution magnetic field data acquired from GOES 10, 11, and 12 over a 2 year period from 2007 and 2008 and examine the local time distribution of EMIC wave events. Unlike previous studies, which reported high EMIC wave occurrence in the postnoon sector with a peak around 1500–1600 magnetic local time (MLT) during magnetically disturbed times (i.e., storm and/or substorm), we observed that quiet time EMIC waves mostly occur in a region from morning (∼0600 MLT) to afternoon (∼1600 MLT) with a peak around 1100–1200 MLT. To investigate whether the quiet time EMIC wave occurrence has a causal relationship with magnetospheric convection enhancement or solar wind dynamic pressure variations, we performed a superposed epoch analysis of solar wind parameters (solar wind speed, density, dynamic pressure, and interplanetary magnetic field Bz) and geomagnetic indices (AE and SYM‐H). From the superposed epoch analysis we found that solar wind dynamic pressure variation is a more important parameter than AE and SYM‐H for quiet time EMIC wave occurrence.

Although we have demonstrated that compressional Pc3-4 pulsations exist in the low-L magnetosphere and that they give rise to pulsations on the ground, we conclude that further study is required to distinguish between the cavity and evanescent modes.

The present Letter puts forth a possible explanation for the outstanding problem of measured proton temperature anisotropy in the solar wind at 1 AU apparently being regulated by the mirror and oblique fire-hose instabilities. Making use of the fact that the local magnetic field intensity near 1 AU undergoes intermediate-scale temporal variations, the present Letter carries out the quasilinear analysis of the temperature anisotropy-driven instabilities with a time-varying local B field, assuming arbitrary initial temperature ratios and parallel betas. It is found that the saturated states in (β(∥), T(⊥)/T(∥)) space are bounded by the mirror and oblique fire-hose instabilities, which is superficially similar to the observation.

We have examined relativistic electron flux losses at geosynchronous orbit under quiet geomagnetic conditions. One 3 day period, from 11 to 13 October 2007, was chosen for analysis because geomagnetic conditions were very quiet (3 day average of K p < 1), and significant losses of geosynchronous relativistic electrons were observed. During this interval, there was no geomagnetic storm activity. Thus, the loss processes associated with geomagnetic field modulations caused by ring current buildup can be excluded. The >2 MeV electron flux at geosynchronous orbit shows typical diurnal variations with a maximum near noon and a minimum near midnight for each day. The flux level of the daily variation significantly decreased from first day to third day for the 3 day period by a factor of >10. The total magnetic field strength (B T ) of the daily variation on the third day, however, is comparable to that on the first day. Unlike electron flux decreases, the flux of protons with energies between 0.8 and 4 MeV adiabatically responses to the daily variation of B T . That is, there is no significant decrease of the proton flux when the electron flux decreases. During the interval of quiet geomagnetic conditions, well-defined electromagnetic ion cyclotron (EMIC) waves were detected at geosynchronous spacecraft. Low-altitude polar-orbiting spacecraft observed the precipitation of energetic protons and relativistic electrons in the interval of EMIC waves enhancement. From these observations, we suggest that the EMIC waves at geosynchronous orbit cause pitch angle scattering and relativistic electron losses to the atmosphere under quiet geomagnetic conditions.

We have statistically studied the relationship between electromagnetic ion cyclotron (EMIC) waves and cold plasmaspheric plasma (Nsp) in the L range of 6–12 using the Time History of Events and Macroscale Interactions during Substorms (THEMIS) data for 2008–2011. The important observational results are as follows: (1) Under quiet geomagnetic conditions (Kp ≤ 1), the maximum occurrence rate of the hydrogen (H) band EMIC waves appears in the early morning sector (0600–0900 MLT) at the outermost region (L= 10–12). (2) Under moderate and disturbed conditions (Kp ≥ 2), the H‐band occurrence rate is higher in the morning‐to‐early‐afternoon sector for L > 10. (3) The high‐occurrence region of helium (He) band waves for Kp ≤ 1 varies from L = 7 to 12 in radial distances along the local time (i.e., at L ∼ 7 near noon and at L= 8–12 near late afternoon). (4) The He‐band waves for Kp ≥ 2 are mainly localized between 1200 and 1800 MLT with a peak around 1500–1600 MLT at L= 8–10. (5) Nsp is much higher for the He‐band intervals than for the H‐band intervals by a factor of 10 or more. The He‐band high occurrence appears at a steep Nsp gradient region. (6) The morning‐afternoon asymmetry of the normalized frequency seen both in H‐band and He‐band is similar to the asymmetric distribution of Nsp along the local time. These observations indicate that the cold plasma density plays a significant role in determining the spectral properties of EMIC waves. We discuss whether a morning‐afternoon asymmetry of the EMIC wave properties can be explained by the spatial distribution of cold plasmaspheric plasma.

We statistically examined the plasmapause location (L pp) under quiet geomagnetic conditions (Kp ≤ 1) using the electron density inferred from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft potential for 2 year period (2008 and 2009). Five hundred forty-three L pp samples were identified under steady quiet conditions with Kp values ≤ 1 during 12 h prior to the plasmapause crossing. From our large data set, we determined the medians and means of L pp in L and magnetic local time (MLT). They are located near geosynchronous orbit and nearly circular. The L pp medians show a slight bulge located in postdusk sector. Comparing with previous models, our median or mean L pp is extended ∼1-2 L from the Earth than the model L pp along the local time from 0800 to 2400 MLT. That is, L pp locations in the previous models are underestimated during quiet geomagnetic conditions.

Recently, Lee and Lysak (1999) suggested that Pi2 pulsations in the inner magnetosphere originate from plasmaspheric virtual resonances (PVR). The PVR model predicts that Pi2 pulsations are not strictly localized to the plasmasphere. Until now there have been no spacecraft observations of the PVR mode outside the plasmasphere. Motivated by the theoretical work, we examine Pi2 pulsations simultaneously observed by the Polar satellite outside the plasmapause and at the low‐latitude Kakioka (L = 1.3) station. We identify 14 events to have high coherence (> 0.7) between the compressional component (bz) in space and the horizontal component (H) at Kakioka during a 2‐month period in 1997. The H‐bz cross phase is ∼180° regardless of the satellite's distance from the plasmapause. The amplitude of the Pi2 pulsations tends to decrease with the distance. On the basis of these observations, we conclude that the PVR mode is an appropriate model for our Pi2 events observed outside the plasmasphere.

[1] The traveltimes of interplanetary (IP) shocks at 1 AU associated with coronal mass ejections (CMEs) can be predicted by the empirical shock arrival (ESA) model of Gopalswamy et al. [2004] based on a constant IP acceleration. We evaluate the ESA model using 91 IP shocks identified from sudden commencement (SC)/sudden impulse (SI) on the Earth and by examining the solar wind data from the ACE and WIND satellites during the period of 1997 to 2002. Out of 91 CME-IP shock pairs, 55 events ($60%) were predicted within ±12 hours from the ESA model. The ESA model predicted $59% (43 out of 73) of the events during solar maximum (1999)(2000)(2001)(2002) and $67% (12 out of 18) of the events during solar minimum (1997)(1998) within ±12 hours from the predicted curve. Comparing the predicted (T mod ) and observed (T obs ) shock arrival times during solar maximum, we find that the deviations (DT = T obs À T mod ) of shock arrival times from the ESA model strongly correlate with the CME initial speeds (V CME ) (linear correlation, r = 0.77). Such a strong correlation indicates that the constant IP acceleration in the ESA model is not reasonably well applied for all V CME . From the linear regression analysis, we obtain a linear fit to the relationship (r = À0.62) between IP shock traveltime T (in hours) and V CME (in kilometer per second) during the solar maximum, which can be expressed as T = 76.86 À 0.02V CME . In addition, we find that the IP shocks associated with the fast CMEs corresponding to strong SC/SI events have short traveltimes compared with other fast CMEs and that there is a negative correlation between the SC/SI strength and the IP shock traveltime. We suggest that this negative correlation is due to not only the V CME but also the CME mass/density and discuss the influence of the mass/density of CME on the arrival time of IP shock at 1 AU.