Postnoon Sector Research Articles

Global Distribution of Discrete and Continuous Spectrum Equatorial Magnetosonic Waves Observed by the Van Allen Probe‐A

AbstractMagnetosonic (MS) waves can either be discrete or continuous in the time‐frequency spectrogram. Through the observation from Van Allen Probe‐A spacecraft, the global distribution of discrete and continuous spectrum MS waves in the region L ∼ 3–6 is determined. According to our research, the continuous spectrum MS waves are greater in amplitude than discrete spectrum MS waves. Also, the impact of changes in geomagnetic activity on continuous spectrum MS waves is much more significant than discrete spectrum MS waves. Besides, it is also demonstrated that the strong discrete spectrum MS waves have a peak wave amplitude of ∼60 pT and an occurrence rate of ∼50% at the noon sector outside the plasmapause. In comparison, the strong continuous spectrum MS waves have a peak wave amplitude of ∼300 pT and an occurrence rate of ∼60% at the postnoon sector inside the plasmapause.

Unique Banded Structures of Plasmaspheric Hiss Waves in the Earth's Magnetosphere

AbstractPlasmaspheric hiss is an electromagnetic wave mode that occurs ubiquitously in the high‐density plasmasphere and contributes crucially to the dynamic behavior of the Earth's Van Allen radiation belts. While plasmaspheric hiss is commonly considered to be a broadband emission with frequencies from ∼100 Hz to several kHz, here we report Van Allen Probes measurements of unambiguous banded signatures of plasmaspheric hiss, uniquely characterized by an upper band above ∼200 Hz, a lower band below ∼100 Hz and a power gap in between. Banded plasmaspheric hiss occurs with the probability ∼8% in the postnoon sector within 2.5–5.0 Earth radii, showing strong dependence on geomagnetic and solar wind conditions. Observations also suggest that banded hiss waves result possibly from two combined sources, the upper band originating from the transformation of chorus waves propagating from outside the plasmasphere, and the lower band from localized excitation inside the plasmasphere, which however requires future investigation. The banded hiss waves shed new light on the evolution of the Earth's radiation belts and have implications for understanding whistler‐mode waves in planetary magnetospheres.

The High‐Latitude Dawn‐Dusk Asymmetry of Ionospheric Plasma Distribution in the Northern Hemisphere

The high-latitude dawn-dusk asymmetrical distribution of the Mean Total Electron Content (MTEC) in the northern hemisphere was statistically investigated using Global Navigation Satellite System (GNSS) vertical TEC data in 2014. In the geomagnetic latitude (MLAT)/geomagnetic local time (MLT) coordinate system, the local time of 0 hr ≤ MLT < 12 hr is defined as the dawn, and 12 hr ≤ MLT < 24 hr is defined as the dusk at MLAT is greater than 70°. The sunlit high-density plasma at lower latitudes in the postnoon sector is the primary source of the asymmetrical MTEC distribution that the duskside MTEC is larger than the dawnside. Except for summer, the diurnal variations of the dusk-dawn difference (DMTEC) and dusk-dawn ratio (DDR) have two peaks and two valleys. The DMTEC and DDR also have significant seasonal variation, which is the greatest in spring (2.9TECu and 22.2%) and the smallest in summer (1.0TECu and 9.2%). The DMTEC and DDR depend significantly on the Kp index and the direction of interplanetary magnetic field (IMF). Except for the DMTEC in spring, the DMTEC and DDR are larger during disturbed times than during quiet times. Except for IMF Bz > 0 in summer, the DMTEC and DDR are larger when IMF By < 0 than those when IMF By > 0, especially in spring and winter.

Van Allen Probes Observations of Symmetric Stormtime Compressional ULF Waves.

Previous spacecraft studies showed that stormtime poloidal ultralow‐frequency (ULF) waves in the ring current region have an antisymmetric (second harmonic) mode structure about the magnetic equator. This paper reports Van Allen Probes observations of symmetric ULF waves in the postnoon sector during a moderate geomagnetic storm. The mode structure is determined from the presence of purely compressional magnetic field oscillations at the equator accompanied by strong transverse electric field perturbations. Antisymmetric waves were also detected but only very late in the recovery phase. The symmetric waves were detected outside the plasmasphere at L = 3.0–5.5 and had peak power at 4–10 mHz, lower than the frequency of the local fundamental toroidal standing Alfvén wave. During the wave events, the flux of protons was enhanced at energies below ∼5 keV, which appears to be a prerequisite for the waves. The protons may provide free energies to waves through drift resonance instability or drift compressional instability, which occur in the presence of radial gradients of plasma parameters.

Van Allen Probes Observations of Oxygen Ions at the Geospace Plume

The geospace plume couples the ionosphere, plasmasphere, and magnetosphere from sub-auroral regions to the magnetopause, on polar field lines, and into the magnetotail. We describe Van Allen Probes observations of ionospheric O+ ions at altitudes of 3–6 RE in the near vicinity of the geospace plume in the noon and post-noon sector. The temporal variation of warm ion fluxes observed as a function of time on a moving spacecraft is complicated by changing spacecraft position and complex ion drift paths and velocities that are highly sensitive to ion energy, pitch angle and L value. In the “notch” region of lower density plasma outside the morning-side plasmapause, bi-directionally field aligned fluxes of lower energy (&amp;lt;5 keV) ions, following corotation-dominated drift trajectories from the midnight sector, are excluded from geospace plume field lines as they are deflected sunward in the plume flow channel. In general, O+ at ring current energies (∼10 keV) is bi-directionally field aligned on plume field lines, while lower energy O+ (&amp;lt;3 keV) are absent. The observation of ion plumes with energies increasing from ∼1 keV–&amp;gt;20 keV in the dusk sector outer plasmasphere is interpreted as evidence for localized ionospheric O+ outflow at the outer edge of the geospace plume with subsequent O+ acceleration to &amp;gt;50 keV in &amp;lt;30 min during the ions’ sunward drift.

Spatial characteristics of recurrent ionospheric storms at low latitudes during solar minimum

Low-latitude ionospheric disturbances with amplitudes of 20 TECU occurred during moderate recurrent magnetic storms were analyzed in time period from 2007 to 2008. The analysis of these so-called recurrent ionospheric storms (RISs) was based on global ionospheric maps of vertical total electron content derived from the ground based GPS network. It was found that positive RISs were developed predominantly in the noon and postnoon sector and they have very large latitudinal and longitudinal extensions of up to 70 and 160°, respectively. Negative RISs occur mainly in the evening sector and they have ∼1.5 time smaller latitudinal and longitudinal extensions. The different location and spatial scales can be explained by different origins of the ionospheric storms. The large-scale positive ionospheric storms are generated by the mechanism of prompt penetration electric field, operating on the dayside, and by equatorward neutral wind disturbance. The negative storms can be related to the mechanism of disturbance dynamo electric field, operating in the evening sector, and to the changing of the thermospheric neutral composition, operating on sunlight side.

A Statistical Study of Spatial Variation of Relativistic Electron Precipitation Energy Spectra With Polar Operational Environmental Satellites

AbstractThe mechanisms that drive relativistic electron precipitation (REP) from the radiation belts can be better understood with a better knowledge of the particle energies involved. National Oceanic and Atmospheric Administration Polar Operational Environmental Satellites, being a network of multiple satellites, can provide multiple point spectral data over a long time period, including the Van Allen Probe's era. The number of energy channels is limited, but the particle detectors on Polar Operational Environmental Satellites have a narrow field of view allowing an investigation of bounce loss cone particles. We use the ratio of count rates in the E3 (&gt;300 keV) and the P6 (&gt;700 keV) channels as a parameter to define spectral hardness. Using this parameter, the spatial variation of spectral hardness of REP events was investigated. It was found that very soft events were mostly found in the dusk‐midnight‐early morning magnetic local time sectors and L∼ 5–7 while the hardest events were located in the postnoon sector peaking at L∼ 4–5. The hardest events peaked at lower L shells, and less than 20% were coincident with low‐energy (30–80 keV) proton precipitation. Further, around 70% of nightside REP coincident with proton precipitation was associated with stretched magnetic field lines indicating that curvature scattering may have been an important driver. Around 62% of nightside REP coincident with proton precipitation associated with relaxed magnetic field lines, suggesting a mechanism other than magnetic field curvature scattering, was highly energetic.

Survey of the Favorable Conditions for Magnetosonic Wave Excitation

AbstractThe ratio of the proton ring velocity (VR) to the local Alfven speed (VA), in addition to proton ring distributions, plays a key factor in the excitation of magnetosonic waves at frequencies between the proton cyclotron frequency fcp and the lower hybrid resonance frequency fLHR in the Earth's magnetosphere. Here we investigate whether there is a statistically significant relationship between occurrences of proton rings and magnetosonic waves both outside and inside the plasmapause using particle and wave data from Van Allen Probe‐A during the time period of October 2012 to December 2015. We also perform a statistical survey of the ratio of the ring energy (ER, corresponding to VR) to the Alfven energy (EA, corresponding to VA) to determine the favorable conditions under which magnetosonic waves in each of two frequency bands (fcp &lt; f ≤ 0.5 fLHR and 0.5 fLHR &lt; f &lt; fLHR) can be excited. The results show that the magnetosonic waves in both frequency bands occur around the postnoon (12–18 magnetic local time, MLT) sector outside the plasmapause when ER is comparable to or lower than EA, and those in lower‐frequency bands (fcp &lt; f ≤ 0.5 fLHR) occur around the postnoon sector inside the plasmapause when ER/EA &gt; ~9. However, there is one discrepancy between occurrences of proton rings and magnetosonic waves in low‐frequency bands around the prenoon sector (6–12 MLT) outside the plasmapause, which suggests either that the waves may have propagated during active time from the postnoon sector after being excited during quiet time, or they may have locally excited in the prenoon sector during active time.

The occurrence and wave properties of EMIC waves observed by the Magnetospheric Multiscale (MMS) mission

AbstractElectromagnetic ion cyclotron (EMIC) waves can precipitate the ring current ions and relativistic electrons and heat the cold electrons in the magnetosphere. This requires comprehensive knowledge of the occurrence and wave properties of EMIC waves. In the present study, we used the data from one new mission, the Magnetospheric Multiscale (MMS) mission launched in March 2015, to investigate the occurrence and wave properties of H+‐band and He+‐band EMIC waves in the magnetosphere. Our statistical results show the following: (1) H+‐band EMIC waves mostly occur in the higher L‐shells (L &gt; 5) while He+‐band EMIC waves are mostly observed in the lower L‐shells (L &lt; 6). (2) The occurrence rate of H+‐band EMIC waves in the dayside is higher than that in the nightside. The highest peak of occurrence rate of H+‐band EMIC waves is in the postnoon sector (5–8 L‐shells), and the secondary peak lies in the small area of the dawn sector. (3) The wave power spectral density peaks in the postnoon and predusk sectors, while the wave normal angles are largest in the dawn sector. (4) Linear and right‐hand polarized H+‐band EMIC waves are mainly in the regions of peak occurrence, while linear polarized waves are seen to also dominate outside of the regions of peak occurrence. The highest occurrence rate of linear polarized He+‐band EMIC waves is observed in the dawn sector. We discussed the results and compared with previous findings.

A statistical study of proton pitch angle distributions measured by the Radiation Belt Storm Probes Ion Composition Experiment

AbstractA statistical study of ring current‐energy proton pitch angle distributions (PADs) in Earth's inner magnetosphere is reported here. The data are from the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) on board the Van Allen Probe B spacecraft from 1 January 2013 to 15 April 2015. By fitting the data to the functional form sinnα, where α is the proton pitch angle, we examine proton PADs at the energies 50, 100, 180, 328, and 488 keV in the L shell range from L = 2.5 to L = 6. Three PAD types are classified: trapped (90° peaked), butterfly, and isotropic. The proton PAD dependence on the particle energy, magnetic local time (MLT), L shell, and geomagnetic activity are analyzed in detail. The results show a strong dependence of the proton PADs on MLT. On the nightside, the n values outside the plasmapause are clearly lower than those inside the plasmapause. At higher energies and during intense magnetic activity, nightside butterfly PADs can be observed at L shells down to the vicinity of the plasmapause. The averaged n values on the dayside are larger than on the nightside. A maximum of the averaged n values occurs around L = 4.5 in the postnoon sector (12–16 MLT). The averaged n values show a dawn‐dusk asymmetry with lower values on the dawnside at high L shells, which is consistent with previous studies of butterfly PADs. The MLT dependence of the proton PADs becomes more distinct with increasing particle energy. These features suggest that drift shell splitting coupled with a radial flux gradient play an important role in the formation of PADs, particularly at L &gt; ~ 4.5.

Orientation variation of dayside auroral arc alignments obtained from all-sky observation at yellow river station, Svalbard

The orientations of dayside auroral arc alignments were calculated for over 40,000 images from all-sky observation at Yellow River Station, Svalbard. For each arc, its “orientation” and “tilt” are defined as the angle the arc alignment makes with the dusk-dawn direction and the local east-west direction, respectively. The mean arc orientation increases linearly with the increasing magnetic local time (MLT). There is a reversal point of the arc tilt located at near 10.5 MLT. Compared with the mean orientation, auroral arc alignment tilts to morning side in the higher latitude and tilts to evening side in the lower latitude in the prenoon sector, whereas it is the opposite in the postnoon sector. We further studied the effects of the interplanetary magnetic field (IMF) on the location of the arc tilt reversal point. We found that the reversal position shifts toward the midday for negative By.

EMIC waves observed at geosynchronous orbit under quiet geomagnetic conditions (Kp ≤ 1)

AbstractWe statistically study the local time distribution of the helium band electromagnetic ion cyclotron (EMIC) waves observed at geosynchronous orbit when geomagnetic activity was low (Kp ≤ 1). In order to identify the geosynchronous EMIC waves, we use high time resolution magnetic field data acquired from GOES 10, 11, and 12 over a 2 year period from 2007 and 2008 and examine the local time distribution of EMIC wave events. Unlike previous studies, which reported high EMIC wave occurrence in the postnoon sector with a peak around 1500–1600 magnetic local time (MLT) during magnetically disturbed times (i.e., storm and/or substorm), we observed that quiet time EMIC waves mostly occur in a region from morning (∼0600 MLT) to afternoon (∼1600 MLT) with a peak around 1100–1200 MLT. To investigate whether the quiet time EMIC wave occurrence has a causal relationship with magnetospheric convection enhancement or solar wind dynamic pressure variations, we performed a superposed epoch analysis of solar wind parameters (solar wind speed, density, dynamic pressure, and interplanetary magnetic field Bz) and geomagnetic indices (AE and SYM‐H). From the superposed epoch analysis we found that solar wind dynamic pressure variation is a more important parameter than AE and SYM‐H for quiet time EMIC wave occurrence.

Modeling the storm time behavior of the magnetosonic waves using solar wind parameters

AbstractWe build empirical models of magnetosonic waves (also called as equatorial noise) both outside and inside the plasmapause, based on the Time History of Events and Macroscale Interactions during Substorms fast‐survey data during the period from 1 May 2010 to 30 November 2014, which utilize solar wind measurements as model inputs, i.e., interplanetary magnetic field (IMF) Bz, solar wind speed (VSW), and dynamic pressure (PSW). For model development, the time delay of magnetosonic wave amplitude relative to the solar wind parameters is first identified based on correlation analysis. We find that PSW is better correlated with the variation of wave amplitude outside the plasmapause than VSW, while the opposite is true inside the plasmapause. Among the solar wind parameters, the preceding southward IMF Bz with the time delay of 3–4 h and 4–5 h, corresponding to outside and inside the plasmapause, respectively, yields the best correlation with the variation of wave amplitude. In this study, we use the solar wind parameter‐based model to investigate magnetosonic wave activity during corotating interaction region‐driven storm. The results show that with increasing southward IMF Bz, wave activity is amplified mostly on the dayside both outside and inside the plasmapause and high amplitude is more shifted toward the prenoon sector, while increasing PSW, it is amplified only in the postnoon sector outside the plasmapause. Our statistical model can explain the observed amplitudes by 38% on average outside the plasmapause and 26% inside, and their model performance is generally higher than that of a model using AE index only as an input.

On the collocation of the cusp aurora and the GPS phase scintillation: A statistical study

AbstractThe climatology map of the GPS phase scintillation identifies two regions of high scintillation occurrences: around magnetic noon and around magnetic midnight. The scintillation occurrence rate is higher around noon, while the scintillation level is stronger around magnetic midnight. This paper focuses on the dayside scintillation region. In order to resolve the role of the cusp auroral processes in the production of irregularities, we put the GPS phase scintillation in the context of the observed auroral morphology. Results show that the occurrence rate of the GPS phase scintillation is highest inside the auroral cusp, regardless of the scintillation strength and the interplanetary magnetic field (IMF). On average, the scintillation occurrence rate in the cusp region is about 5 times as high as in the region immediately poleward of it. The scintillation occurrence rate is higher when the IMF Bz is negative. When partitioning the scintillation data by the IMF By, the distribution of the scintillation occurrence rate around magnetic noon is similar to that of the poleward moving auroral form (PMAF): there is a higher occurrence rate at earlier (later) magnetic local time when the IMF By is positive (negative). This indicates that the irregularities which give rise to scintillations follow the IMF By‐controlled east‐west motion of the aurora and plasma. Furthermore, the scintillation occurrence rate is higher when IMF By is positive when the cusp is shifted toward the post noon sector where it may get easier access to the higher density plasma. This suggests that the combined auroral activities (e.g., PMAF) and the density of the intake solar EUV ionized plasma are crucial for the production of scintillations.

Excitation of dayside chorus waves due to magnetic field line compression in response to interplanetary shocks

AbstractThe excitation of magnetospheric whistler‐mode chorus in response to interplanetary (IP) shocks is investigated using wave data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft. As an example, we show a typical chorus wave excitation following an IP shock event that was observed by THEMIS in the postnoon sector near the magnetopause on 3 August 2010. We then analyze characteristic changes during this event and perform a survey of similar events during the period 2008–2014 using the THEMIS and OMNI data set. Our statistical analysis demonstrates that the chorus wave excitation/intensification in response to IP shocks occurs only at high L shells (L &gt; 8) on the dayside. We analyzed the variations of magnetic curvature following the arrival of the IP shock and found that IP shocks lead to more homogeneous background magnetic field configurations in the near‐equatorial dayside magnetosphere; and therefore, the threshold of nonlinear chorus wave growth is likely to be reduced, favoring chorus wave generation. Our results provide the observational evidence to support the concept that the geomagnetic field line configuration plays a key role in the excitation of dayside chorus.

Large‐amplitude GPS TEC variations associated with Pc5–6 magnetic field variations observed on the ground and at geosynchronous orbit

AbstractLarge‐amplitude variations in GPS total electron content (TEC) at Pc5–6 (&lt;6.67 mHz) frequencies have been observed, using a high data rate Global Positioning System (GPS) receiver of the Canadian High Arctic Ionospheric Network. TEC variations with peak‐to‐peak amplitudes of 2–7 TEC units (1 TECU = 1016 el m−2) were observed over a 2.5 h period in the postnoon sector on 9 September 2011, during a period of high auroral activity within a moderate geomagnetic storm. TEC observations were from the Sanikiluaq, Nunavut (56.54°N, 280.77°E) GPS receiver located in the auroral region. Over this same time period, compressional Pc5–6 magnetic field variations were observed by the geosynchronous GOES 13 magnetometer and the ground‐based Sanikiluaq magnetometer. GOES 13 has a northern magnetic footprint in close proximity to Sanikiluaq. Cross‐correlation analysis indicates that magnetic field and TEC variations were possibly linked. No natural hazards or nuclear explosions capable of exciting TEC perturbations were reported on this day. Using a triangulation technique involving TEC measurements of multiple GPS satellites, the propagation velocity of TEC variations in the ionosphere was also calculated. This calculation revealed two distinct events: lower frequency (~0.9 mHz) TEC variations that propagated westward, consistent with the westward propagation of compressional Pc5 waves observed by GOES 13 and 15 satellites, and higher‐frequency (~3.3 mHz) TEC variations that propagated southward. This is the first report of variations in ionospheric TEC linked to satellite observations of Pc5–6 ULF waves.

Magnetic flux circulation in the rotationally driven giant magnetospheres

AbstractThe giant‐planet magnetodiscs are shaped by the radial transport of plasma originating in the inner magnetosphere. Magnetic flux transport is a key aspect of the stretched magnetic field configuration of the magnetodisc. While net mass transport is outward (ultimately lost to the solar wind), magnetic flux conservation requires a balanced two‐way transport process. Magnetic reconnection is a critical aspect of the balanced flux transport. We present a comprehensive analysis of current sheet crossings in Saturn's magnetosphere using Cassini magnetometer data from 2004 to 2012 in an attempt to quantify the circulation of magnetic flux, emphasizing local time dependence. A key property of flux transport is the azimuthal bend forward or bend back of the magnetic field. The bend back configuration is an expected property of the magnetodisc with net mass outflow, but the bend forward configuration can be achieved with the rapid inward motion of mostly empty flux tubes following reconnection. We find a strong local time dependence for the bend forward cases, localized mostly in the postnoon sector, indicating that much of the flux‐conserving reconnection occurs in the subsolar and dusk sector. We suggest that the reconnection occur in a complex and patchy network of reconnection sites, supporting the idea that plasma can be lost on small scales through a “drizzle”‐like process. Auroral implications for the observed flux circulation will also be presented.

THEMIS Na exosphere observations of Mercury and their correlation with in-situ magnetic field measurements by MESSENGER

The Na exosphere of Mercury is being studied since its discovery in mid ‘80s from Earth-based telescopes, and it has revealed a high dynamics and variability. Although the processes and their relationships characterising the Hermean exosphere generation and dynamics are still not exhaustively understood, there are no doubts on a tight interconnection among the planet's surface, exosphere, intrinsic magnetic field, the solar wind and the Interplanetary Magnetic Field (IMF). In this paper we analyze an extended dataset of images of the exospheric Na emission, collected from 2009 to 2013, by means of the THEMIS ground-based telescope, in order to perform a comprehensive statistical study of the recurrent Na emission patterns, and also their potential relationship with the IMF variability. For this purpose, we take advantage of a subset (years 2011–2013) of contemporary in-situ measurements of the IMF obtained by the MAG instrument on-board the MESSENGER spacecraft. We found that the high latitude double peak is the most common Na emission pattern, supporting the view that the solar wind ion precipitation through the polar cusps has an important role in the generation of the observed Na exospheric configuration. Moreover, the lack of a statistically significant North–South asymmetry seems to disfavor the existence of an asymmetric and/or shifted intrinsic magnetic dipole. By analyzing a subset of quasi-full disk images, we found that the double peak Na emission is typically aligned along the meridian, mostly occurring in the pre-noon sector (53%), about 1/3 close to the noon meridian (36%), whereas only 11% takes place in the post-noon sector. Finally, the comparison with the IMF data seems to indicate that the contribution of the IMF BX component to the magnetic reconnection is generally weak, even if we found a noticeable correlation between positive IMF BX and symmetric double peak pattern. Negative IMF BZ values are usually connected to double peak emission, whereas positive IMF BZ values are more frequently associated to single peaked equatorial Na emission.

The dayside magnetopause location during radial interplanetary magnetic field periods: Cluster observation and model comparison

Abstract. The present work has investigated the midlatitudinal magnetopause locations under radial interplanetary field (RIMF) conditions. Among 262 (256) earthward (sunward) RIMF events from years of 2001 to 2009, Cluster satellites have crossed the magnetopause 22(12) times, with 10 (7) events occurring at midlatitudes. The observed midlatitudinal magnetopause positions are compared with two empirical magnetopause models (Shue et al., 1998; Boardsen et al., 2000) (hereafter referred to as the Shue98 model and the Boardsen00 model). The observation–model differences exhibit local time asymmetry. For earthward RIMF cases, the Shue98 model underestimates the magnetopause positions in the postnoon sector, while it overestimates the magnetopause positions in the dawn and dusk sectors. The Boardsen00 model generally underestimates the magnetopause after 6 MLT (magnetic local time), with larger deviations in the postnoon sector as compared to those in the prenoon. For sunward RIMF cases, the selected events are mainly clustered around the dawn and dusk sectors. The comparison with the Shue98 model indicates contractions in the dawn and expansions in the dusk sector, while the comparison with Boardsen00 indicates general expansions, with larger expansions in the later local time sectors. The local time variations in the differences between observations and the Shue98 and the Boardsen00 models indicate that the real magnetopause could be asymmetrically shaped during radial IMF periods, which should be considered by magnetopause models. The observation–model differences in the magnetopause positions (Δ RMP) during RIMF periods correlate well with the solar wind dynamic pressure, with larger Δ RMP for larger Pd. The southern magnetopause expands further outward relative to the model prediction when the dipole tilt angle is more negative (local summer in the Southern Hemisphere). For earthward RIMF cases, the generally good correlations between Δ RMP and the IMF cone angle are consistent with the previous hypothesis (Dušík et al., 2010) that, with more radial IMF, the subsolar magnetopause will expand further outward, owever, this is not the case for the comparison with Boardsen00 during sunward IMF periods, as it shows less dependence on the IMF cone angle.

Asymmetric Kelvin‐Helmholtz propagation at Saturn's dayside magnetopause

AbstractAt Saturn's magnetopause, the shear flows are maximized (minimized) in the prenoon (postnoon) sector due to the rapid planetary rotation and the corotating magnetodisc. As such, the prenoon sector is expected to be more Kelvin‐Helmholtz (KH) unstable than the postnoon sector; however, in situ Cassini data analyses showed that the evidence of KH activity favors the postnoon sector. In this study, we use a two‐dimensional MHD simulation to demonstrate that fast‐growing KH modes strongly deform and diffuse the boundary layer on a time scale of a few minutes in the prenoon sector. Therefore, the KH observational signature is difficult to identify by spacecraft in the diffused boundary layer. KH vortices originating in the subsolar region (roughly from 10 to 14 local times) are transported to the postnoon sector and the wavelength is enlarged due to the gradient of shear flow, which is a plausible reason why KH events are more often observed in the postnoon sector. The prediction of the local boundary normal direction distribution as a function of spacecraft inward/outward crossing in the postnoon sector suggested by our simulation is qualitatively consistent with Cassini in situ observational results. We also discuss the impact of this dawn‐dusk asymmetric Kelvin‐Helmholtz evolution on magnetic reconnection at Saturn's magnetopause boundary.