Even moderate geomagnetic pulsations can cause fluctuations of <i>fo</i>F2 frequency of the auroral ionosphere

The ionosonde at the Sodankylä Geophysical Observatory (SOD; 67∘ N, 27∘ E; Finland) routinely performs vertical sounding once per minute which enables the study of fast ionospheric variations at a frequency of the long-period geomagnetic pulsations Pc5–6/Pi3 (1–5 mHz). Using the ionosonde data from April 2014–December 2015 and colocated geomagnetic measurements, we have investigated a correspondence between the magnetic field pulsations and variations of the critical frequency of radio waves reflected from the ionospheric F2 layer (foF2). For this study, we have developed a technique for automated retrieval of the critical frequency of the F2 layer from ionograms. As a rule, the Pc5–6/Pi3 frequency band fluctuations in foF2 were observed at daytime during quiet or moderately disturbed space weather conditions. In most cases (about 80 %), the coherence between the foF2 variations and geomagnetic pulsations was low. However in some cases (specified as “coherent”) the coherence was as large as γ2≥0.5. The following conditions are favorable for the occurrence of coherent cases: enhanced auroral activity (6 h maximal auroral electrojet (AE) ≥800 nT), high solar wind speed (V>600 km/s), fluctuating solar wind pressure and northward interplanetary magnetic field. In the cases when the coherence was higher at shorter periods of oscillations, the magnetic pulsations demonstrated features typical for the Alfvén field line resonance.

Interaction of Pc4 ULF waves with solar wind velocity and its dependence on Kp values

Background/Objectives: Magnetic Pulsations recorded on the ground in the earth are produced by processes inside the magnetosphere and solar wind. These processes produce a wide variety of ULF hydromagnetic wave type which can be categorized on the ground as either Pi or Pc pulsations (irregular or continuous). Methods: Distinctive regions of the magnetosphere originate different frequencies of waves. Digital Dynamic Spectra (DDS) for the northsouth (X), east-west (Y) and vertical (Z) components of the recorded data were constructed for every day for 365 days (January 1 to December 31, 2005) in the station order PON, HAN and NAG respectively. Pc4 geomagnetic pulsations are quasi-sinusoidal fluctuations in the earth’s magnetic field in the length range 45-150 seconds. The magnitude of these pulsations ranges from fraction of a Nano Tesla (nT) to several nT. The monthly variation of Pc4 occurrence has a Kp dependence range of 0 to 9-. However, Pc4 occurrence was reported for Kp values, yet the major Pc4 events occurred for rage 5+ <Kp< 8+. The magnitudes of intervals of Pc4 occurrence decreased in the station order PON, HAN and NAG respectively. Analysis of the data for the whole year 2005 provided similar patterns of Pc4 occurrence for Vsw at all the three stations. Although Pc4 ULF wave occurrence become reported for Vsw ranging from 250 to 1000 Km/s, yet the major Pc4 event recorded for a Vsw range of 300-700 Km/sec. Findings: The current study is undertaken for describing the interaction of Pc4 ULF waves with solar wind speed and its dependence on Kp values. The results suggest that the solar wind control Pc4 occurrence through a mechanism in which Pc4 wave energy is convected through the magnetosheath and coupled to the standing oscillations of the magnetospheric field lines. PACS Nos: 94.30.cq; 96.50.Tf Keywords: Geomagnetic micropulsations; MHD waves and instabilities; Solar wind-control of Pc4 pulsation

Foreshock wave transmission into the magnetosheath and magnetosphere: results from global hybrid-Vlasov simulations

&lt;p&gt;The foreshock, extending upstream of the quasi-parallel shock and populated with shock-reflected particles, is home to intense wave activity in the ultra-low frequency range.&lt;em&gt; &lt;/em&gt;The most commonly observed of these waves are the &amp;#8220;30 s&amp;#8221; waves, fast magnetosonic waves propagating sunward in the plasma rest frame, but carried earthward by the faster solar wind flow. These waves are thought to be the main source of Pc3 magnetic pulsations (10 &amp;#8211; 45 s) in the dayside magnetosphere. A handful of case studies with suitable spacecraft conjunctions have allowed simultaneous investigations of the wave properties in different geophysical regions, but the global picture of the wave transmission from the foreshock through the magnetosheath into the magnetosphere is still not known. In this work, we use global simulations performed with the hybrid-Vlasov model Vlasiator to study the Pc3 wave properties in the foreshock, magnetosheath and magnetosphere for different solar wind conditions. We find that in all three regions the wave power peaks at higher frequencies when the interplanetary magnetic field strength is larger, consistent with previous studies. While the transverse wave power decreases with decreasing Alfv&amp;#233;n Mach number in the foreshock, the compressional wave power shows little variation. In contrast, in the magnetosheath and the magnetosphere, the compressional wave power decreases with decreasing Mach number. Inside the magnetosphere, the distribution of wave power varies with the IMF cone angle. We discuss the implications of these results for the propagation of foreshock waves across the different geophysical regions, and in particular their transmission through the bow shock.&lt;/p&gt;

Martian Aquifers: Detection by Magnetic Sounding from Surface Magnetometry

&lt;p&gt;Magnetic pulsations are observed at Mars in the magnetotail and on the surface by the Maven and InSight magnetometers. The surface observations exhibit a frequency-dependent polarization in which the amplitude of the vertical component weakens with increasing frequency. This frequency dependence is not seen in the source regions studied by MAVEN. The source of the frequency dependence must be in the subsurface of Mars. The attenuation is consistent with an aquifer that is 3 km thick, containing water of the conductivity of terrestrial seawater.&lt;/p&gt;

Ground-Space Observations of Pc5 Poleward Moving Auroral Arc Pulsations and Field-Line Resonances in the Post-Midnight Sector: THEMIS Observations

We investigate the Pc5 poleward moving auroral arc (PMAA) pulsations (~ 4–5 min period) using the ground-based all-sky imager network and the Time History of Events and Macroscale Interactions during Substorms (THEMIS) A, D, and E satellites, whose footprints were located near the PMAA in the post-midnight sector. The Pc5 PMAA pulsations considered herein occurred in conjunction with the enhancement of the magnetic and electric field oscillations observed near the equatorial plane of the magnetosphere. The magnetospheric oscillation signal displayed three-cycle oscillations, which correspond primarily to the PMAA pulsations. The value of coherence between the magnetospheric oscillations and the luminosity pulsations was higher than 0.9. Based on these observations, it is suggested that the PMAA pulsations and the magnetospheric field oscillations are initiated by the same physical mechanism and thus oscillate concurrently by the magnetosphere-ionosphere (M-I) coupling. The satellite data indicated a longer period of magnetospheric oscillations at the higher latitude site. On the other hand, the measured period of the PMAA pulsation was almost constant in the lower latitude region (~ 68.5°-70.0° MLAT), whereas in the higher latitude region (~ 70.0°-70.5° MLAT) it increased with increasing latitude. This signature demonstrates that the oscillations on the lower latitudinal side of the PMAA conformed with the monochromatic frequency field-line resonance (FLR) where the oscillation period was constant and independent of latitude, whereas the higher latitude side of the PMAA presented a multi-frequency FLR region where the period lengthened with increasing latitude. The Pc5 magnetic pulsations observed on the ground neither exhibited a clear coincidence with the PMAA pulsations nor with the magnetospheric magnetic oscillations. On the other hand, the H component of magnetic pulsations demonstrated a rather similar behavior to that of the ion pressure variation within the magnetosphere. The solar wind speed was significantly high, approximately 650 km/s, during this event. The magnetospheric magnetic and electric field oscillations could be triggered simultaneously in a wide region by an impulse such as rapid convection changes caused by the sudden variations of the interplanetary magnetic field (IMF) Bz, which was observed by the SuperDARN radar and the Geotail satellite.

Periodic modulation of the upper ionosphere by ULF waves as observed by SuperDARN radars and GPS/TEC technique

&lt;p&gt;We compare the simultaneous magnetometer, SuperDARN radar, and GPS observations during Pc5 wave event on March 02, 2002. A possible correspondence between those instruments may help to determine the mechanism of the ionosphere modulation by magnetospheric disturbances. Transient Pc5 pulsations (2.6 mHz) in the morning sector, stimulated by the solar wind density jumps, have been detected simultaneously by ground magnetometers and the Kodiak and King Salmon SuperDARN radars.&amp;#160; Besides that, pulsations with the same periodicity have been found in the rate of total electron content (TEC), dTEC/dt (ROT), variations in several GPS radio paths. The ratio between the spectral amplitudes of the Doppler velocities and magnetic pulsations (X component) on the ground are Vx/Bx~7-12 (m/s)/nT and Vy/Bx~27 (m/s)/nT. The ratio between the oscillation amplitudes of ROT and ionospheric Doppler meridional (Vx) and azimuthal (Vy) velocities are ROT/Vx~0.02-0.07 (dTECu/min)/(m/s) and ROT/Vy~0.004 (dTECu/min)/(m/s). The correspondence between simultaneous periodic variations of the ionospheric Doppler velocity and geomagnetic field can be reasonably well interpreted quantitively on the basis of theory of Alfven wave interaction with the thin ionospheric layer. However, order-of-magnitudes estimates of possible TEC modulation mechanisms show that a responsible mechanism which can interpret the observed ratios has not been found yet.&amp;#160;&lt;/p&gt;

Magnetic Pulsations and Transients on Martian Surface

&lt;p&gt;InSight is the first Mars surface mission that includes a magnetometer, and one of the first discoveries made by the InSight FluxGate (IFG) magnetometer is the ultra-low-frequency (ULF) waves, or magnetic pulsations, on the Martian surface. By studying magnetic pulsations and transient signatures in more than six months of IFG data, we find that the morphologies of these two types of perturbations have considerable variations from their counterparts on the Earth, reflecting the fundamental differences between the magnetospheres with and without a global magnetic field. The most noticeable ULF waves are the continuous pulsations (Pc) occurring at around midnight and with wave periods of the order of 100 sec, or in the Pc 4 frequency band when the terminology of terrestrial magnetic pulsations is used. Broadband pulsations at Pc 5 frequencies (i.e., a few mHz) have also been observed. Comparisons with lander activities and InSight&amp;#8217;s Temperature and Wind for InSight Subsystem (TWINS) data confirm that the observed magnetic pulsations are not caused by tremors of the lander. Simultaneous observations by MAVEN in the solar wind and InSight on Mars indicate that the upstream waves in front of Mars bow shock can hardly reach the dayside surface, leading to a dearth of magnetic pulsations in the daytime. In addition, solar wind discontinuities or transient events can induce noticeable surface magnetic responses only in the nightside, suggesting that the magnetic pileup region and ionosphere can effectively shield external magnetic disturbances. MAVEN observations also help identify sources of magnetic pulsations seen on the Martian surface. While the low-frequency, broad-band Pc 5 pulsations may be excited by the oscillations on the flanks of the induced magnetosphere associated with solar wind variations or the Kelvin-Helmholtz instability, there is a strong indication that the nightside Pc 4 pulsations on the surface originate from the compressional oscillations in the magnetotail. Different from the flow-generated fast mode waves in the terrestrial magnetotail, the fast mode in the Martian magnetotail could travel toward the planet without substantial coupling to the Alfv&amp;#233;n mode. The Mars-propagating fast mode experiences little reflection from the ionosphere and can produce surface magnetic pulsations at low latitudes on the nightside. These first findings of magnetic pulsations and transients on the Martian surface not only reveal the origins and propagation of magnetic signals from the outer space but also help determine the source model for the magnetic sounding of Mars' interior.&lt;/p&gt;

Long-term observation of magnetic pulsations through the ELF Hylaty station located in the Bieszczady Mountains (south–eastern Poland)

Ground-based measurements of ultra- and extremely low-frequency waves (ULF/ELF) carried out in 2005–2016 (the 23rd and 24th solar cycle) at the ELF Hylaty station in Bieszczady Mountains (south–eastern Poland) were used to identify the days (360 days) in which magnetic pulsation events (MPEs) occurred. To reveal sources of MPEs at the Sun we considered their correlation with the basic indices describing solar activity. Our analysis, like earlier studies, did not reveal a significant positive correlation between the MPE detection rate and the sunspot numbers (SSN). On the other hand, we showed that MPEs are strongly correlated (correlation coefficient ≈0.70) with moderate (Dst < −70 nT) and intense (Dst < −100 nT) geomagnetic disturbances expressed by the Disturbance Storm Index (Dst). We recognized all sources of these geomagnetic storms associated with the considered MPEs. Only 44% of the MPEs were associated with storms caused by CMEs listed in the CDAW LASCO CME catalog. 56% of the MPEs were associated with storms caused by other phenomena including corotating interaction regions (CIRs), slow solar wind or CMEs not detected by LASCO. We also demonstrated that the CMEs associated with the MPEs were very energetic, i.e. they were extremely wide (partial and halo events) and fast (with the average speed above 1100 km s−1). CMEs and CIRs generally appear in different phases of solar cycles. Because MPEs are strongly related to both of these phenomena they cannot be associated with any phase of a solar cycle or with any indicator characterizing a 11-year solar activity. We also suggested that the low number of MPEs associated with CMEs is due to the anomalous 24 solar cycle. During this cycle, due to low density of the interplanetary medium, CMEs could easily eject and expand, but they were not geoeffective.