scholarly journals Magnetic field and dynamic pressure ULF fluctuations in coronal-mass-ejection-driven sheath regions

2013 ◽  
Vol 31 (9) ◽  
pp. 1559-1567 ◽  
Author(s):  
E. K. J. Kilpua ◽  
H. Hietala ◽  
H. E. J. Koskinen ◽  
D. Fontaine ◽  
L. Turc
Keyword(s):  
Magnetic Field ◽  
Dynamic Pressure ◽  
Low Frequency ◽  
Leading Edge ◽  
Interplanetary Coronal Mass Ejections ◽  
Abrupt Decrease ◽  
Superposed Epoch Analysis ◽  
Sheath Region ◽  
Epoch Analysis ◽  
The Imf

Abstract. Compressed sheath regions form ahead of interplanetary coronal mass ejections (ICMEs) that are sufficiently faster than the preceding solar wind. The turbulent sheath regions are important drivers of magnetospheric activity, but due to their complex internal structure, relatively little is known on the distribution of the magnetic field and plasma variations in them. In this paper we investigate ultra low frequency (ULF) fluctuations in the interplanetary magnetic field (IMF) and in dynamic pressure (Pdyn) using a superposed epoch analysis of 41 sheath regions observed during solar cycle 23. We find strongest fluctuation power near the shock and in the vicinity of the ICME leading edge. The IMF and Pdyn ULF power have different profiles within the sheath; the former is enhanced in the leading part of the sheath, while the latter is increased in the trailing part of the sheath. We also find that the ICME properties affect the level and distribution of the ULF power in sheath regions. For example, sheath regions associated with strong or fast ICMEs, or those that are crossed at intermediate distances from the center, have strongest ULF power and large variation in the power throughout the sheath region. The weaker or slower ICMEs, or those that are crossed centrally, have in general considerably weaker ULF power with relatively smooth profiles. The strong and abrupt decrease of the IMF ULF power at the ICME leading edge could be used to distinguish the ICME from the preceding sheath plasma.

scholarly journals On the alignment of velocity and magnetic fields within magnetosheath jets

2019 ◽  
Author(s):  
Ferdinand Plaschke ◽  
Maria Jernej ◽  
Heli Hietala ◽  
Laura Vuorinen
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Entire Range ◽  
Dynamic Pressure ◽  
Bow Shock ◽  
Superposed Epoch Analysis ◽  
Study Results ◽  
Epoch Analysis ◽  
The Magnetic Field

Abstract. Jets in the subsolar magnetosheath are localized enhancements in dynamic pressure that are able to propagate all the way from the bow shock to the magnetopause. Due to their excess velocity with respect to their environment, they push slower ambient plasma out of their way, creating a vortical plasma motion in and around them. Simulations and case study results suggest that jets also modify the magnetic field in the magnetosheath on their passage, aligning it more with their velocity. Based on MMS jet observations and corresponding superposed epoch analyses of the angles φ between the velocity and magnetic fields, we can confirm that this suggestion is correct. However, the effect is small: Typically, reductions in φ of only 10° are observed at jet core regions, where the jets' velocities are largest. Furthermore, time series of angles φ pertaining to individual jets significantly deviate from the superposed epoch analysis results. They usually exhibit large variations over the entire range of φ: 0° to 90°. This variability is commonly somewhat larger within jets than outside, masking the systematic decrease in φ at core regions of individual jets.

scholarly journals On the alignment of velocity and magnetic fields within magnetosheath jets

2020 ◽  
Vol 38 (2) ◽  
pp. 287-296
Author(s):  
Ferdinand Plaschke ◽  
Maria Jernej ◽  
Heli Hietala ◽  
Laura Vuorinen
Keyword(s):  
Magnetic Fields ◽  
Entire Range ◽  
Dynamic Pressure ◽  
Bow Shock ◽  
Superposed Epoch Analysis ◽  
Magnetospheric Multiscale ◽  
Study Results ◽  
Epoch Analysis ◽  
The Magnetic Field

Abstract. Jets in the subsolar magnetosheath are localized enhancements in dynamic pressure that are able to propagate all the way from the bow shock to the magnetopause. Due to their excess velocity with respect to their environment, they push slower ambient plasma out of their way, creating a vortical plasma motion in and around them. Simulations and case study results suggest that jets also modify the magnetic field in the magnetosheath on their passage, aligning it more with their velocity. Based on Magnetospheric Multiscale (MMS) jet observations and corresponding superposed epoch analyses of the angles ϕ between the velocity and magnetic fields, we can confirm that this suggestion is correct. However, while the alignment is more significant for faster than for slower jets, and for jets observed close to the bow shock, the overall effect is small: typically, reductions in ϕ of around 10∘ are observed at jet core regions, where the jets' velocities are largest. Furthermore, time series of ϕ pertaining to individual jets significantly deviate from the superposed epoch analysis results. They usually exhibit large variations over the entire range of ϕ: 0 to 90∘. This variability is commonly somewhat larger within jets than outside them, masking the systematic decrease in ϕ at core regions of individual jets.

scholarly journals On the response of ionospheric electrojets to solar wind discontinuities

2009 ◽  
Vol 27 (10) ◽  
pp. 3791-3803 ◽  
Author(s):  
M. Palmroth ◽  
T. I. Pulkkinen ◽  
J. Polvi ◽  
A. Viljanen ◽  
P. Janhunen
Keyword(s):  
Solar Wind ◽  
Dynamic Pressure ◽  
Pressure Change ◽  
Solar Wind Dynamic Pressure ◽  
Simultaneous Increase ◽  
Sudden Increase ◽  
Step Size ◽  
Ionospheric Response ◽  
Superposed Epoch Analysis ◽  
The Imf

Abstract. We investigate the ionospheric response to solar wind discontinuities as detected by the IE index computed from IMAGE ground magnetometers. The solar wind discontinuities include both sudden increases as well as decreases of the solar wind dynamic pressure, recorded by the SWEPAM instrument of the ACE spacecraft during the period 1998–2004. In our statistical study, we identify four categories of events: 1) sudden increases of the dynamic pressure with a simultaneous increase of the interplanetary magnetic field (IMF) magnitude; 2) sudden increases of the dynamic pressure accompanied with a simultaneous decrease of the IMF; 3) sudden decreases of the dynamic pressure accompanied with a sudden increase of the IMF; and 4) sudden decreases of the dynamic pressure with relatively steady IMF. We perform a superposed epoch analysis for the four event categories to distinguish the ionospheric response. We find that the IE index increases/decreases in response to the solar wind dynamic pressure increases/decreases regardless of the simultaneous change in the IMF or the amount of estimated input energy. We investigate the magnitude of the ionospheric response according to the IMF north-south direction, the dynamic pressure step size as well as the pressure level prior the dynamic pressure change. We find that the ionospheric result is augmented for larger pressure steps, while the prior IMF has a role only in some of the event categories. We also perform global MHD simulation runs to investigate the ionospheric dissipation rate during such solar wind discontinuities, and find that the simulation results are in good qualitative accordance with the observational statistical results.

scholarly journals The Pulsating Magnetosphere at Jupiter

2020 ◽  
Author(s):  
Harry Manners ◽  
Adam Masters
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Dynamic Pressure ◽  
Wave Spectrum ◽  
Low Frequency ◽  
Global Scale ◽  
Wave Activity ◽  
Ulf Waves ◽  
Ulf Wave ◽  
Wide Range

<p>The magnetosphere of Jupiter is the largest planetary magnetosphere in the solar system, and plays host to internal dynamics that remain, in many ways, mysterious. Prominent among these mysteries are the ultra-low-frequency (<strong>ULF</strong>) pulses ubiquitous in this system. Pulsations in the electromagnetic emissions, magnetic field and flux of energetic particles have been observed for decades, with little to indicate the source mechanism. While ULF waves have been observed in the magnetospheres of all the magnetized planets, the magnetospheric environment at Jupiter seems particularly conducive to the emergence of ULF waves over a wide range of periods (1-100+ minutes). This is mainly due to the high variability of the system on a global scale: internal plasma sources and a powerful intrinsic magnetic field produce a highly-compressible magnetospheric cavity, which can be reduced to a size significantly smaller than its nominal expanded state by variations in the dynamic pressure of the solar wind. Compressive fronts in the solar wind, turbulent surface interactions on the magnetopause and internal plasma processes can also all lead to ULF wave activity inside the magnetosphere.</p><p>To gain the first comprehensive view of ULF waves in the Jovian system, we have performed a heritage survey of magnetic field data measured by six spacecraft that visited the magnetosphere (Galileo, Ulysses, Voyager 1 & 2 and Pioneer 10 & 11). We found several-hundred wave events consisting of wave packets parallel or transverse to the mean magnetic field, interpreted as fast-mode or Alfvénic MHD wave activity, respectively. Parallel and transverse events were often coincident in space and time, which may be evidence of global Alfvénic resonances of the magnetic field known as field-line-resonances. We found that 15-, 30- and 40-minute periods dominate the Jovian ULF wave spectrum, in agreement with the dominant “magic frequencies” often reported in existing literature.</p><p>We will discuss potential driving mechanisms as informed by the results of the heritage survey, how this in turn affects our understanding of energy transfer in the magnetosphere, and potential investigations to be made using data from the JUNO spacecraft. We will also discuss the potential for multiple resonant cavities, and how the resonance modes of the Jovian magnetosphere may differ from those of the other magnetized planets.</p>

scholarly journals Magnetopause energy and mass transfer: results from a global MHD simulation

2006 ◽  
Vol 24 (12) ◽  
pp. 3467-3480 ◽  
Author(s):  
M. Palmroth ◽  
T. V. Laitinen ◽  
T. I. Pulkkinen
Keyword(s):  
Magnetic Field ◽  
Mass Transfer ◽  
Solar Wind ◽  
Energy Transfer ◽  
Dynamic Pressure ◽  
Solar Wind Dynamic Pressure ◽  
Closed Field ◽  
Dayside Magnetopause ◽  
Solar Wind Parameters ◽  
The Imf

Abstract. We use the global MHD model GUMICS-4 to investigate the energy and mass transfer through the magnetopause and towards the closed magnetic field as a response to the interplanetary magnetic field (IMF) clock angle θ=arctan (BY/BZ), IMF magnitude, and solar wind dynamic pressure. We find that the mass and energy transfer at the magnetopause are different both in spatial characteristics and in response to changes in the solar wind parameters. The energy transfer follows best the sin2 (θ/2) dependence, although there is more energy transfer after large energy input, and the reconnection line follows the IMF rotation with a delay. There is no clear clock angle dependence in the net mass transfer through the magnetopause, but the mass transfer through the dayside magnetopause and towards the closed field occurs preferably for northward IMF. The energy transfer occurs through areas at the magnetopause that are perpendicular to the subsolar reconnection line. In contrast, the mass transfer occurs consistently along the reconnection line, both through the magnetopause and towards the closed field. Both the energy and mass transfer are enhanced in response to increased solar wind dynamic pressure, while increasing the IMF magnitude does not affect the transfer quantities as much.

Radial evolution of magnetic field fluctuations in an ICME sheath

2020 ◽  
Author(s):  
Simon Good ◽  
Matti Ala-Lahti ◽  
Erika Palmerio ◽  
Emilia Kilpua ◽  
Adnane Osmane
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Leading Edge ◽  
Permutation Entropy ◽  
The Sun ◽  
Interplanetary Coronal Mass Ejections ◽  
Field Fluctuations ◽  
Fluctuation Amplitude ◽  
Radial Evolution

<p>The sheaths of compressed solar wind that precede interplanetary coronal mass ejections (ICMEs) commonly display large-amplitude magnetic field fluctuations. As ICMEs propagate radially from the Sun, the properties of these fluctuations may evolve significantly. We present a case study of an ICME sheath observed by a pair of radially aligned spacecraft at around 0.5 and 1 AU from the Sun. Radial changes in fluctuation amplitude, compressibility, inertial-range spectral slope, permutation entropy, Jensen-Shannon complexity, and planar structuring are characterised.  We discuss the extent to which the observed evolution in the fluctuations is similar to that of solar wind emanating from steady sources at quiet times, how the evolution may be influenced by evolving local factors such as leading-edge shock orientation, and how the perturbed heliospheric environment associated with ICME propagation may impact the evolution more generally.</p>

scholarly journals High-intensity, long-duration, continuous AE activity events associated with Alfvénic fluctuations in 2003

2017 ◽  
Vol 35 (6) ◽  
pp. 1231-1240 ◽  
Author(s):  
Alan Prestes ◽  
Virginia Klausner ◽  
Arian Ojeda-González
Keyword(s):  
Magnetic Field ◽  
High Intensity ◽  
High Speed ◽  
Cross Correlation ◽  
Alfven Waves ◽  
Alfvén Waves ◽  
Superposed Epoch Analysis ◽  
Fast Speed ◽  
Long Duration ◽  
Epoch Analysis

Abstract. The interaction between a fast-speed and a low-speed stream causes large-amplitude Alfvénic fluctuations; consequently, the intermittency and the brief intervals of southward magnetic field associated with Alfvén waves may cause high levels of AE activity, the so-called high-intensity, long-duration, continuous AE activity (HILDCAA). In this article, the 4 h windowed Pearson cross-correlation (4WPCC) between the solar wind velocity and the interplanetary magnetic field (IMF) components is performed in order to confirm that the less strict HILDCAA (HILDCAAs*) events include a larger number of Alfvén waves than the HILDCAA events, once HILDCAAs disregard part of the phenomenon. Actually, a HILDCAA event is entirely contained within a HILDCAA* event. However, the opposite is not necessarily true. This article provides a new insight, since the increase of Alfvén waves results in an increase of auroral electrojet activity; consequently, it can cause HILDCAAs* events. Another important aspect of this article is that the superposed epoch analysis (SEA) results reaffirm that the HILDCAAs* are associated with high-speed solar streams (HSSs), and also the HILDCAAs* present the same physical characteristics of the traditional HILDCAA events.

scholarly journals Ionospheric and geomagnetic responses to changes in IMF <i>B<sub>Z</sub></i>: a superposed epoch study

1997 ◽  
Vol 15 (2) ◽  
pp. 217-230 ◽  
Author(s):  
C. J. Davis ◽  
M. N. Wild ◽  
M. Lockwood ◽  
Y. K. Tulunay
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Dynamic Pressure ◽  
Wind Pressure ◽  
Solar Wind Dynamic Pressure ◽  
High Latitudes ◽  
Winter Solstice ◽  
Ionospheric Response ◽  
Dst Index ◽  
The Imf

Abstract. Superposed epoch studies have been carried out in order to determine the ionospheric response at mid-latitudes to southward turnings of the interplanetary magnetic field (IMF). This is compared with the geomagnetic response, as seen in the indices Kp, AE and Dst. The solar wind, IMF and geomagnetic data used were hourly averages from the years 1967–1989 and thus cover a full 22-year cycle in the solar magnetic field. These data were divided into subsets, determined by the magnitudes of the southward turnings and the concomitant increase in solar wind pressure. The superposed epoch studies were carried out using the time of the southward turning as time zero. The response of the mid-latitude ionosphere is studied by looking at the F-layer critical frequencies, foF2, from hourly soundings by the Slough ionosonde and their deviation from the monthly median values, δfoF2. For the southward turnings with a change in Bz of δBz > 11.5 nT accompanied by a solar wind dynamic pressure P exceeding 5 nPa, the F region critical frequency, foF2, shows a marked decrease, reaching a minimum value about 20 h after the southward turning. This recovers to pre-event values over the subsequent 24 h, on average. The Dst index shows the classic storm-time decrease to about –60 nT. Four days later, the index has still to fully recover and is at about –25 nT. Both the Kp and AE indices show rises before the southward turnings, when the IMF is strongly northward but the solar wind dynamic pressure is enhanced. The average AE index does register a clear isolated pulse (averaging 650 nT for 2 h, compared with a background peak level of near 450 nT at these times) showing enhanced energy deposition at high latitudes in substorms but, like Kp, remains somewhat enhanced for several days, even after the average IMF has returned to zero after 1 day. This AE background decays away over several days as the Dst index recovers, indicating that there is some contamination of the currents observed at the AE stations by the continuing enhanced equatorial ring current. For data averaged over all seasons, the critical frequencies are depressed at Slough by 1.3 MHz, which is close to the lower decile of the overall distribution of δfoF2 values. Taking 30-day periods around summer and winter solstice, the largest depression is 1.6 and 1.2 MHz, respectively. This seasonal dependence is confirmed by a similar study for a Southern Hemisphere station, Argentine Island, giving peak depressions of 1.8 MHz and 0.5 MHz for summer and winter. For the subset of turnings where δBz > 11.5 nT and P ≤ 5 nPa, the response of the geomagnetic indices is similar but smaller, while the change in δfoF2 has all but disappeared. This confirms that the energy deposited at high latitudes, which leads to the geomagnetic and ionospheric disturbances following a southward turning of the IMF, increases with the energy density (dynamic pressure) of the solar wind flow. The magnitude of all responses are shown to depend on δBz. At Slough, the peak depression always occurs when Slough rotates into the noon sector. The largest ionospheric response is for southward turnings seen between 15–21 UT.

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