Occurrence characteristics of relativistic electron microbursts from SAMPEX observations

Abstract

We study the occurrence of relativistic microbursts observed by the Solar Anomalous Magnetospheric Particle Explorer (SAMPEX) satellite. An algorithm is used to identify 193,694 relativistic microbursts in the > 1.05 MeV electron fluxes occurring across the time period 23 August 1996 to 11 August 2007, nearly a full solar cycle. Our observations are normalized to provide the change in absolute occurrence rates with various parameters. We find that relativistic microbursts are mostly confined to the outer radiation belt, from L = 3–8, occurring primarily on the morningside, between 0 and 13 magnetic local time (MLT). This L and MLT distribution is consistent with the L and MLT distribution of whistler mode chorus amplitude. Thus, our observations favor whistler mode chorus wave activity as a driver of relativistic microbursts. Relativistic microbursts become more frequent as the geomagnetic activity level increases and are more frequent during equinoxes than during the solstices. The peak occurrence frequency of the relativistic microbursts moves to lower L as the geomagnetic activity increases, reaching a peak occurrence rate of one microburst every 10.4 s (on average) at L = 4 for 6.6 ≤ Kp ≤ 8.7. Microbursts primarily occur outside of the plasmapause and track the inward movement of the plasmapause with increasing geomagnetic activity. The L and MLT distribution of the relativistic microbursts exhibits a peak occurrence of one microburst every 8.6 (98.0) s during active (disturbed) conditions, with the peak located at L = 5 (L = 5.5) and 08 (08) MLT.

Characteristics of Relativistic Microburst Intensity From SAMPEX Observations

“…We now investigate how the mean intensity of relativistic microbursts changes under different levels of geomagnetic activity. In order to remain consistent with earlier work (e.g., Li et al, ; Douma et al, ), we have defined three levels of geomagnetic activity based on AE * (the mean of AE over the previous 1 hr); quiet corresponds to AE * ≤ 100 nT, disturbed corresponds to 100 < AE * ≤ 300 nT, and active corresponds to AE * > 300 nT. However, our large relativistic microburst data set allows us to also extend these well used AE * ranges to include higher geomagnetic activity; intense conditions correspond to AE * > 550 nT and extreme conditions correspond to AE * > 750 nT.…”

“…As we move outward from the plasmapause in Figure a we observe the largest mean intensity for the relativistic microbursts occurs at 1.25 L from the plasmapause with a mean intensity of 1,484 (MeV cm 2 sr s) −1 . This L band is ∼1 L closer to the plasmapause than the L band with the most frequent microburst activity (Douma et al, ). Moving inward from the plasmapause we note that the microburst intensity appears to increase.…”

Relativistic Electron Microburst Events: Modeling the Atmospheric Impact

“…In order to describe the characteristic precipitation in these events, we utilize the relativistic microburst data set derived from Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) Heavy Ion Large Telescope (HILT), recently reported in Douma et al (). We employ the O'Brien et al () algorithm which was updated by Blum et al () to include the microburst intensity.…”

Comparison of Relativistic Microburst Activity Seen by SAMPEX With Ground‐Based Wave Measurements at Halley, Antarctica

“…SAMPEX observed the relativistic microburst event while at an average IGRF L of 5.8 (the event was seen from L = 5.3–6.3). Figure a presents the > 1 MeV flux observed by SAMPEX during the time of this microburst event, with the microburst algorithm triggers (described in more detail in Douma et al, ) indicated by the red crosses. This microburst event consists of 16 individual microbursts detected by the algorithm, occurring during an A E index value of 298 nT ( D s t of −11 nT, and K p of 3).…”