Conjugate Observations of Quasiperiodic Emissions by the Van Allen Probes Spacecraft and Ground‐Based Station Kannuslehto

Abstract

Whistler mode waves observed in the Earth's inner magnetosphere at frequencies between about 0.5 and 4 kHz which exhibit a nearly periodic time modulation of the wave intensity are called quasiperiodic (QP) emissions. Conjugate measurements of QP events at several different locations can be used to estimate their spatial extent and spatiotemporal variability. Results obtained using conjugate QP measurements provided by the ground-based station Kannuslehto (L ≈ 5.5) and the Van Allen Probes spacecraft (L shells between about 1.1 and 6.5) between September 2012 and November 2017 are presented. Altogether, 26 simultaneously detected events were analyzed. The event modulation periods and frequency-time structures were generally the same at all observation points. Spatial separations of the spacecraft and the ground-based station during conjugate observations are typically within about 40 • in azimuth and from about 1 to 3 in L shell. RBSP consistently observes events at lower L shells than Kannuslehto, with the event occurrence primarily inside of the plasmasphere. Ratios of Poynting fluxes observed by the spacecraft and on the ground are used to evaluate event intensity variations related to the spacecraft position. It is found that the intensity decreases considerably both at low L shells and outside of the plasmasphere. Finally, an event containing a gap in its frequency-time structure related to a sudden change of its properties is analyzed in detail.

Multipoint Observations of Quasiperiodic Emission Intensification and Effects on Energetic Electron Precipitation

“…The QP emissions were coherently modulated at the locations of the two probes during almost the entire period (08:45–12:00 UT), suggesting that they either originated from a common source or were modulated locally by a common mechanism. The separation of the two satellites during this period was ∆MLT = 0.6–1.3 h and ∆ L < 1.3, which is within the typical range of the QP emission coherence scale (Bezděková et al, 2020).…”

“…Many previous studies reported coherently modulated QP emissions that were simultaneously observed at conjugate points along the magnetic field line (Carson et al., 1965; Martinez‐Calderon et al., 2016, 2020; Titova et al., 2015). QP emissions can also be coherent across magnetic field lines, and multipoint observations from the ground (Carson et al., 1965) and equatorial satellites (Bezděková et al., 2020; Li et al., 2017; Němec et al., 2016) recorded coherent QP emissions across several hours in local time and several L‐shells. QP emissions had been used to study the propagation of whistler‐mode waves by tracing the waves with quasiperiodic rising‐tone structures (e.g., Němec et al., 2014).…”

Quasiperiodic Emissions and Related Particle Precipitation Bursts Observed by the DEMETER Spacecraft

“…Wave vector directions close to vertical are needed on the bottom of the ionosphere in order to allow the wave propagation to the ground. This is consistent with conjugate observations of the emissions by spacecraft and ground-based instruments, which indeed reveal the emissions to extend to lower L-shells on board the spacecraft than on the ground (Bezděková et al, 2020).…”

“…The observed oblique wave normal angles demonstrate that the wave propagation is primarily unducted (Martinez‐Calderon et al., 2016; Němec et al., 2018). The unducted propagation is believed to be responsible for the same QP modulation being observed over comparatively large regions of space (Bezděková et al., 2020; Němec, Bezděková, et al., 2016; Němec, Hospodarsky, et al., 2016; Němec, Santolík, Parrot, et al., 2013). This is supported by multipoint measurements and detailed time delay analysis, which reveals a time delay on the order of seconds between different locations (Martinez‐Calderon et al., 2016; Němec et al., 2014).…”

New Type of Short High‐Frequency VLF Patches (“VLF Birds”) Above 4–5 kHz

“…Despite the importance of direct VLF measurements in the space with satellite instruments, the continuous ground-based observations can provide the unique possibility to study the temporal dynamics of the waves. In particular, still now many findings of properties and dynamics of different types of natural VLF emissions such as chorus, hiss, long series of QP emissions, have been found basing on the ground-based observations at Kannuslehto station (KAN) in Northern Finland (e.g., Bezděková et al, 2020;Demekhov et al, 2020;Kleimenova et al, 2015Kleimenova et al, , 2019Manninen et al, , 2014Manninen et al, , 2015Manninen et al, , 2016Manninen et al, , 2017Manninen et al, , 2020Manninen et al, , 2012Martinez-Calderon et al, 2019;Shklyar et al, 2020;Titova et al, 2017Titova et al, , 2015.…”