Conjugate observations of quasiperiodic emissions by the Cluster, Van Allen Probes, and THEMIS spacecraft

Abstract

We present results of a detailed analysis of two electromagnetic wave events observed in the inner magnetosphere at frequencies of a few kilohertz, which exhibit a quasiperiodic (QP) time modulation of the wave intensity. The events were observed by the Cluster and Van Allen Probes spacecraft and in one event also by the THEMIS E spacecraft. The spacecraft were significantly separated in magnetic local time, demonstrating a huge azimuthal extent of the events. Geomagnetic conditions at the times of the observations were very quiet, and the events occurred inside the plasmasphere. The modulation period observed by the Van Allen Probes and THEMIS E spacecraft (duskside) was in both events about twice larger than the modulation period observed by the Cluster spacecraft (dawnside). Moreover, individual QP elements occur about 15 s earlier on THEMIS E than on Van Allen Probes, which might be related to a finite propagation speed of a modulating ULF wave.

Conjugate observations of a remarkable quasiperiodic event by the low‐altitude DEMETER spacecraft and ground‐based instruments

“…Attributing the observed time delay to an azimuthally propagating ULF wave responsible for the QP modulation, we can estimate its azimuthal speed to be about 1.2°/s. This value is consistent with the value of about 1.5–2°/s estimated by Němec et al []. Moreover, this wave would propagate tailward, which is consistent with ground‐based statistical results [ Olson and Rostoker , ; Chisham and Orr , ] and recent satellite observations [ Takahashi et al , , ].…”

“…The observed time delays were on the order of a few seconds, and they were explained by the ray tracing analysis, considering the unducted wave propagation. Conjugate observations of a QP event by the Van Allen Probes and THEMIS spacecraft revealed the time delay as large as 15 s for azimuthal separations of about 25° [ Němec et al , ]. It was argued that such a large time delay cannot be due to the propagation of the QP emissions.…”

“…Multipoint measurements of the same event are crucial to distinguish between spatial and temporal variations of the phenomenon. Such observations are typically rather difficult to gather, and a detailed analysis of only a few such events has been performed up to now [ Němec et al , , , ; Titova et al , ; Němec et al , ]. Moreover, while ground‐based measurements can provide important information about temporal variations of an event [ Smith et al , ; Manninen et al , , , , ], spacecraft measurements are crucial to obtain the information about the event spatial variability.…”

“…However, one should note that although this generation mechanism assumes the presence of a compressional ULF wave, magnetic field pulsations observed on the ground are typically Alfvénic, Multipoint measurements of the same event are crucial to distinguish between spatial and temporal variations of the phenomenon. Such observations are typically rather difficult to gather, and a detailed analysis of only a few such events has been performed up to now [Němec et al, 2013a[Němec et al, , 2013bTitova et al, 2015;Němec et al, 2016]. Moreover, while ground-based measurements can provide important information about temporal variations of an event [Smith et al, 1991;Manninen et al, 2012Manninen et al, , 2013Manninen et al, , 2014aManninen et al, , 2014b, spacecraft measurements are crucial to obtain the information about the event spatial variability.…”

Recent Advancements and Remaining Challenges Associated With Inner Magnetosphere Cross‐Energy/Population Interactions (IMCEPI)

“…The in situ and ground‐based data sets have tremendously grown in the past few years due to active spacecraft missions such as Cluster, Van Allen Probes, and Arase, providing substantial data samples for understanding the inner magnetosphere environment. The ample data sets help derive long‐term trends and evolutions of the physical processes within the inner magnetosphere in response to different driving conditions, by looking into various wave properties (e.g., occurrence rate, amplitudes, spatial sizes, obliquity, and propagation; Artemyev et al, ; Aryan et al, , ; Fu et al, ; Kersten et al, ; W. Li et al, ; W. Li et al, ; Malaspina et al, , ; Meredith et al, ; Nemec et al, ; Ni et al, ; Saikin et al, ; Santolík, Macúsová, et al, ; Spasojevic et al, ; Yue et al, ; Zhima et al, , ), inner magnetosphere plasma compositions (Claudepierre et al, ; Fernandes et al, ; Kistler & Mouikis, ; Kistler, Mouikis, Spence, et al, ; Sarno‐Smith et al, ; Yue et al, ), ion mass density along closed magnetic field lines as well as mass loading in response to geomagnetic activity levels (Sandhu et al, , ), plasma sheet composition outside the inner magnetosphere (e.g., Denton et al, ; Nosé, ), the spacecraft surface charging environment (Sarno‐Smith et al, ), plasmapause model (He et al, ; Liu et al, ; X.‐X. Zhang et al, ), electric field model (Califf et al, ), and global magnetospheric field model (Tsyganenko & Andreeva, ), which upgraded from a series of previous models (e.g., Tsyganenko, , , ; Tsyganenko & Sitnov, ).…”