Occurrence Rates of Electromagnetic Ion Cyclotron (EMIC) Waves With Rising Tones in the Van Allen Probes Data Set

Abstract

In Fourier time‐frequency power spectrograms of satellite magnetic field data, electromagnetic ion cyclotron (EMIC) waves may feature discrete, rising tone structures that rapidly increase in frequency. Using data from the Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) fluxgate magnetometer, we conducted a statistical study of EMIC waves from September 2012 through June 2016. We compared the occurrence rates and spatial distributions for all EMIC waves with those for rising tone EMIC waves as a function of magnetic local time (MLT) and L shell, as well as a function of RXY and Z in solar‐magnetic (SM) coordinates. Overall, EMIC waves occurred during 2.4% of the time period considered, but rising tone EMIC waves were only found during 0.2% of the time period considered. About 7%–8% of the minutes of orbital coverage with H+ or He+ band EMIC waves had rising tones. The regions of peak occurrence rates for H+ and He+ band waves, as well as waves with rising tones, were found in the noon and dusk sectors for 4 < L < 6. The preferred regions for H+ waves as a function of RXY and Z SM suggest an association with magnetospheric compressions near noon and interactions between plumes and the ring current near dusk. Peak occurrence rates for O+ band waves were found between 2 < L < 4 at all MLT, and over a wide range of L shells near dusk. No rising tones were found in the O+ band.

Amplitude‐Dependent Properties and Excitation Mechanisms of EMIC Waves in the Earth's Inner Magnetosphere

“…No matter how large the amplitude is, EMIC waves have higher occurrence rates in the northern hemisphere than that in the southern hemisphere, consistent with the results shown in Sigsbee et al. (2023) which investigates EMIC waves with rising tones. This hemispheric asymmetry feature may be related to the stronger geomagnetic activity when Van Allen Probe B was located in the northern hemisphere, as shown in Figure S2 of Supporting Information .…”

On the Dynamics of Ultrarelativistic Electrons (>2 MeV) Near L* = 3.5 During 8 June 2015

“…The results in Figure 5 show decrease of PSD at lower pitch angles while preserving the populations at higher pitch angles, thus narrowing the pitch angle spectra, as observed during the event and shown in Figure 5. O+ band EMIC waves are less common than He+ band EMIC waves as found by studies of EMIC observations from the Van Allen Probes era (H. Chen et al, 2019;Sigsbee et al, 2023;Yu et al, 2015); however, when O+ EMIC waves are observed, they are most prevalent in the range 3 < L < 4, which is the region where this loss is observed, and are as common as H+ EMIC waves in this region (H. Chen et al, 2019;Saikin et al, 2015). The dwell time of the spacecraft decreases with L value (as shown by H. Chen et al, 2019;Sigsbee et al, 2023;Yu et al, 2015) decreasing the likelihood of observing EMIC wave structures in this region.…”

“…Sigsbee et al. (2023) report EMIC waves are observed ∼2.4% of the time during the Van Allen Probes era, considering data from both probes. Studies by Yu et al.…”

“…PSD minima in this L* region can be difficult to find with automatic detection algorithms such as those used by Drozdov et al (2022) due to low PSD at small L* values. The dwell time of the spacecraft also decreases at low L* value, making the occurrence of these features less likely to be reported (e.g., H. Chen et al, 2019;Saikin et al, 2015;Sigsbee et al, 2023).…”

“…Statistical studies of EMIC wave occurrence during the Van Allen Probes era describe the spatial occurrence and frequency of these waves. Saikin et al (2015) compiled EMIC wave observations using Van Allen Probes data from 2012 to 2015 and Sigsbee et al (2023) studied the same data set until June 2016. Both studies showed that most observations of H+ and He+ band EMIC waves occur between L = 4 and 6, and O+ band waves are mostly observed at L < 2-4.…”