Decrease In Magnetization Research Articles

A Review of Alfvénic Turbulence in High‐Speed Solar Wind Streams: Hints From Cometary Plasma Turbulence

AbstractSolar wind turbulence within high‐speed streams is reviewed from the point of view of embedded single nonlinear Alfvén wave cycles, discontinuities, magnetic decreases (MDs), and shocks. For comparison and guidance, cometary plasma turbulence is also briefly reviewed. It is demonstrated that cometary nonlinear magnetosonic waves phase‐steepen, with a right‐hand circular polarized foreshortened front and an elongated, compressive trailing edge. The former part is a form of “wave breaking” and the latter that of “period doubling.” Interplanetary nonlinear Alfvén waves, which are arc polarized, have a ~180° foreshortened front and with an elongated trailing edge. Alfvén waves have polarizations different from those of cometary magnetosonic waves, indicating that helicity is a durable feature of plasma turbulence. Interplanetary Alfvén waves are noted to be spherical waves, suggesting the possibility of additional local generation. They kinetically dissipate, forming MDs, indicating that the solar wind is partially “compressive” and static. The ~2 MeV protons can nonresonantly interact with MDs leading to rapid cross‐field (~5.5% Bohm) diffusion. The possibility of local (~1 AU) generation of Alfvén waves may make it difficult to forecast High‐Intensity, Long‐Duration AE Activity and relativistic magnetospheric electrons with great accuracy. The future Solar Orbiter and Solar Probe Plus missions should be able to not only test these ideas but to also extend our knowledge of plasma turbulence evolution.

Plasma and Magnetic-Field Characteristics of Magnetic Decreases in the Solar Wind at 1 AU: Cluster-C1 Observations

Magnetic decreases (MDs) are structures observed in interplanetary space with significant decreases in magnetic-field magnitude. Events with little or no change in the field direction are called linear magnetic decreases (LMDs), the others are called nonlinear MDs (NMDs). In this article we focus on LMD and NMD trains, where LMD trains are defined as at least three LMDs in a row and NMD trains as trains (≥ three MDs in a row) that are not all linear. If the temporal separation between two MDs was shorter than five minutes, they were considered as one train event. A total of 16 273 MD trains (including 897 LMD trains and 15 376 NMD trains) were identified and studied. The details of the background magnetic-field and plasma (e.g. ion-density and velocity) features were examined and compared with the average solar-wind properties. LMD trains are found to occur in regions with relatively low magnetic-field strengths, high ion-number densities, and large plasma βs (ratio of the plasma thermal pressure to the magnetic pressure). In sharp contrast, NMD trains have plasma properties similar to the average solar wind. Forty-three LMD trains are related to interplanetary coronal mass ejections (ICMEs) (including 19 events that occurred in ICME sheaths and 24 in the ICME proper), while 222 LMD trains occurred in corotating interaction regions (CIRs), and the remaining 632 events in the normal solar wind. The LMD trains that occurred in ICME sheaths are thought to be associated with the generation mechanism of the mirror-mode instability. Only 552 of the NMD trains are related to ICMEs (including 236 events in ICME sheaths and 316 in ICMEs proper), while 3889 (25 %) NMD trains occurred in CIRs, and the remaining 71 % occurred in the normal solar wind. Because the NMD trains have various plasma properties that differ from the LMD trains, we suggest that NMD trains may be generated by different mechanisms, for instance by a steepening of Alfven waves.

Decréscimos magnéticos no meio interplanetário

Este artigo tem como objetivo apresentar aos leitores os fenômenos conhecidos como decréscimos magnéticos (MDs, do inglês magnetic decreases) ou buracos magnéticos (MHs, do inglês magnetic holes). Tais estruturas são regiões do espaço interplanetário e da própria magnetosfera terrestre onde a magnitude do campo magnético local é diminuída, às vezes drasticamente. Os MDs são transportados pelo vento solar e já foram detectados tanto em altas quanto em baixas latitudes heliosféricas. Suas características são relativamente bem conhecidas. No entanto, ainda não existe um consenso sobre uma única teoria para sua formação. A interação não-ressonante de partículas carregadas com MDs leva à difusão das partículas perpendicularmente ao campo magnético, em consequência do deslocamento dos seus centros de guia.

Generation process of magnetic decreases and the resulting kinetic effects on energetic particles within corotating interaction regions

One‐dimensional hybrid simulations are performed to examine the formation mechanism of magnetic decrease (MD) structures within corotating interaction regions (CIRs) and their effect on the energetic particle profiles associated with the shock acceleration at the CIR boundaries. As the source of MD formation, Alfvén wave structures propagating in the fast solar wind, which make depressions in the magnetic field magnitude after passing through the CIR reverse shock, are taken into account. The observed MD properties of the simultaneous density and pressure enhancement are confirmed. The key process of reducing the field intensity is the incidence of Alfvénic field variations in the anisotropic plasma (P⊥ > P∥) of the reverse shock downstream. Directional changes in the magnetic field induce energy exchange between parallel and perpendicular components, leading to isotropization and adiabatic field reduction. Magnetic mirror points consequently form, where field‐aligned streams further intensify and particles accumulate in MDs. The capture of particles by MDs delays the reverse shock evolution. Therefore, it is expected that MDs will reduce the CIR dimension and lower the efficiency of particle acceleration at CIR shocks.

A computational study of nonresonant cross-field diffusion of energetic particles due to their interaction with interplanetary magnetic decreases

We present a new method of calculating cross-field diffusion of charged particles due to their interactions with interplanetary magnetic decreases (MDs) in high heliospheric latitudes. We use a geometric model that evaluates perpendicular diffusion to the ambient magnetic field as a function of particle's gyroradius, MD radius, ratio between fields outside and inside the MD, and a random impact parameter. We use Ulysses magnetic field data of 1994 to identify the MDs and get the empirical size and magnetic field decrease distribution functions. We let protons with energies ranging from 100 keV to 2 MeV interact with MDs. The MD characteristics are taken from the observational distribution functions using the Monte Carlo method. Calculations show that the increase in diffusion tends to saturate when particles' gyroradius becomes as large as MD radii, and that particles' gyroradius increases faster than diffusion distance as the energy of the particles is increased.

Magnetosheath and heliosheath mirror mode structures, interplanetary magnetic decreases, and linear magnetic decreases: Differences and distinguishing features

[1] There has been considerable confusion in the literature about what mirror mode (MM), magnetic decrease (MD), and linear magnetic decrease (LMD) structures are and are not. We will reexamine past spacecraft observations to demonstrate the observational similarities and differences between these magnetic and plasma structures. MM structures in planetary magnetosheaths, cometary sheaths, and the heliosheath have the following characteristics: (1) the structures have little or no changes in the magnetic field direction across the magnetic dips; (2) the structures have quasiperiodic spacings, varying from ∼20 proton gyroradii (rp) in the Earth's magnetosheath to ∼57 rp in the heliosheath; and (3) the magnetic dips have smooth edges. Magnetosheath MM structures are generated by the mirror instability where β⊥/β∥ > 1 + 1/β⊥ (β is the plasma thermal pressure divided by the magnetic pressure). In general, the sources of free energy for the mirror instability are reasonably well understood: shock compression, field line draping, and, in the cases of comets and the heliosheath, also ion pickup. The observational properties of interplanetary MDs are as follows: (1) there is a broad range of magnetic field angular changes across them; (2) their thicknesses can range from as little as 2–3 rp to thousands of rp, with no “characteristic” size; and (3) they typically are bounded by discontinuities. The mechanism(s) for interplanetary MD generation is (are) currently unresolved, although at least five different mechanisms have been proposed in the literature. Tsurutani et al. (2009a) have argued against mirror instability for those MDs generated within interplanetary corotating interaction regions. Interplanetary LMDs are by definition a subset of MDs with small angular changes across them (θ < 10°). Are LMDs generated by the mirror instability or by another mechanism? Is it possible that there are several different types of LMDs involving different generation mechanisms? At the present time, no one knows the answers to these latter questions.

Magnetic Decreases (MDs) and mirror modes: two different plasma β changing mechanisms

Abstract. We discuss two different physical processes that create localized high β plasma regions. One is nonlinear wave-steepening, generating magnetic decreases (MDs) by a ponderomotive force. The other is the mirror instability generating alternating high and low β plasma regions. It is demonstrated that MDs and mirror modes are observationally quite different structures. MDs spatially occur in interplanetary space and mirror modes primarily in planetary magnetosheaths. MDs are characterized by: 1) variable (exponentially decreasing number with increasing) angular changes, 2) variable (exponentially decreasing) thicknesses, and 3) no characteristic inter-event spacings. In sharp contrast, mirror modes are characterized by: 1) little or no angular changes across the structures, 2) a characteristic scale size, and 3) are quasiperiodic in nature. Arguments are presented for the recently observed magnetic dips in the heliosheath being mirror mode structures. The sources of free energy for instability are discussed. Both structures are important for energetic particle transport in astrophysical and heliospheric plasmas.

Magnetic decrease formation from &lt;1 AU to ∼5 AU: Corotating interaction region reverse shocks

Magnetic decreases (MDs) have been identified and studied throughout a Ulysses fast latitude scan that lasted from 29 February 1992 to 14 September 1993. Ulysses' distance was ∼5 AU from the Sun. MDs were unbiasedly selected by application of the Interplanetary Magnetic Decrease Automatic Detection code. MDs were found to occur in high‐occurrence‐frequency “clusters” with the top 10 peak events varying in magnitude from 116 MDs per day to 36 MDs per day. For comparative purposes, quiet, nonpeak intervals had an occurrence rate of 4.3 MDs per day. Each of the 10 MD clusters was analyzed in detail to determine their solar wind dependences. MD clusters were often found to occur within corotating interaction regions (CIRs), mainly localized in the trailing portions of CIRs between the interface (IF) and the reverse shock (RS). The MD clusters were divided into smaller subclusters. Within the limits of this study, MD subclusters were always found to occur in high‐β (1 &lt; β &lt; 102) regions (HBRs). Small MD subclusters were detected in HBRs downstream of forward shocks (FSs) but less frequently than for the trailing portion of the CIR. The FS to IF region is generally a low‐β region (LBR), where β ≤ 1.0. The 3920 MDs were identified in the study. The temporal thickness distribution of MDs is given by N = 2173 e−(t/17.3), where t is in seconds. The magnetic field angular changes were calculated across MDs. The angular dependence is %MDs = 2 + 48e−(ΔΘ°/18.8°). Only 13.5% of MDs were “linear” with angular changes &lt;10° across the structures. Because MDs are found in abundance in the region spanning the CIR RS to close to the IF, it is argued that MDs must be formed continuously from close to the Sun (r &lt; 0.5 AU) to ∼5 AU. The older MDs (those found close to the IF) are “fossils” that have been convected radially outward to ∼5 AU. A scenario is presented to explain the HBRs downstream of both CIR RSs and quasi‐parallel FSs. The location of MD clusters in the trailing parts of CIRs and the paucity of linear MDs indicate that MD generation by mirror mode instability is unlikely. More promising candidates are shock compression of phase‐steepened Alfvén waves, shock–directional discontinuity interactions, and downstream turbulence. We emphasize the phase‐steepened Alfvén wave mechanism.

The interplanetary magnetic decrease automatic detection (IMDAD) code

Abstract A new code, called the interplanetary magnetic decrease (MD) automatic detection (IMDAD) code, has been developed to enable researchers in the field to rapidly identify MD events for further analyses. The criterion used for MD selection is B min &lt; XB 0, where X is a variable value and B 0 is the ambient magnetic field magnitude. The code can be applied to data sets from different instruments/missions located in different space plasma environments in the heliosphere. The code has been tested during slow solar wind, fast solar wind and CIR intervals at ∼5 AU (from November 28 to December 03, 1992). For this test, we used a sliding window with a width of 300 seconds applied to 1-second high-resolution magnetic field data. The events identified by the code have been confirmed by hand analyses. The routine was able to identify 57 of the 118 MDs identified by hand (∼50%). The selection criteria for IMDAD and hand-analyses MDs were not exactly the same, accounting for the different rates of occurrence. What is particularly encouraging is that IMDAD did not falsely identify any events. The discrepancies between the two methods are discussed in the text. This code will be made available to the general public.

Corotating solar wind streams and recurrent geomagnetic activity: A review

Solar wind fast streams emanating from solar coronal holes cause recurrent, moderate intensity geomagnetic activity at Earth. Intense magnetic field regions called Corotating Interaction Regions or CIRs are created by the interaction of fast streams with upstream slow streams. Because of the highly oscillatory nature of the GSM magnetic field z component within CIRs, the resultant magnetic storms are typically only weak to moderate in intensity. CIR‐generated magnetic storm main phases of intensity Dst &lt; −100 nT (major storms) are rare. The elongated storm “recovery” phases which are characterized by continuous AE activity that can last for up to 27 days (a solar rotation) are caused by nonlinear Alfven waves within the high streams proper. Magnetic reconnection associated with the southward (GSM) components of the Alfvén waves is the solar wind energy transfer mechanism. The acceleration of relativistic electrons occurs during these magnetic storm “recovery” phases. The magnetic reconnection associated with the Alfvén waves cause continuous, shallow injections of plasma sheet plasma into the magnetosphere. The asymmetric plasma is unstable to wave (chorus and other modes) growth, a feature central to many theories of electron acceleration. It is noted that the continuous AE activity is not a series of substorm expansion phases. Arguments are also presented why these AE activity intervals are not convection bays. The auroras during these continuous AE activity intervals are less intense than substorm auroras and are global (both dayside and nightside) in nature. Owing to the continuous nature of this activity, it is possible that there is greater average energy input into the magnetosphere/ionosphere system during far declining phases of the solar cycle compared with those during solar maximum. The discontinuities and magnetic decreases (MDs) associated with interplanetary Alfven waves may be important for geomagnetic activity. In conclusion, it will be shown that geomagnetic storms associated with high‐speed streams/CIRs will have the same initial, main, and “recovery” phases as those associated with ICME‐related magnetic storms but that the interplanetary causes are considerably different.

Rapid evolution of magnetic decreases (MDs) and discontinuities in the solar wind: ACE and Cluster

An experiment was conducted to identify and measure the same magnetic decreases (MDs) and interplanetary discontinuities at two different points in space separated by ∼0.01 AU along the radial distance from the sun. The ACE and Cluster satellite data were used in this study. Solar wind speeds measured at Cluster were used to calculate the approximate (earlier) times of arrival at ACE. The field component changes at both spacecraft were intercompared to ensure that the same discontinuities were detected. Out of 7 MDs/discontinuities selected, 6 were identified at ACE. The MDs/discontinuities were identified within 7 s to 35 s of the convection times. The MD thicknesses at Cluster were substantially smaller than at ACE. The ratio of thicknesses varied from 0.21 to 0.045. One possible interpretation of this observation is that the Alfvén wave front (rotational discontinuity) is steepening by a factor of greater than 15 per wavelength propagated. This may have important consequences for the generation of high frequency turbulence in interplanetary space. It is noted that the 6 discontinuities transited the ∼0.01 AU distance from ACE to Cluster with essentially the solar wind convection speed. A possible mechanism contributing to this feature is discussed.

Nonlinear Alfvén waves, discontinuities, proton perpendicular acceleration, and magnetic holes/decreases in interplanetary space and the magnetosphere: intermediate shocks?

Abstract. Alfvén waves, discontinuities, proton perpendicular acceleration and magnetic decreases (MDs) in interplanetary space are shown to be interrelated. Discontinuities are the phase-steepened edges of Alfvén waves. Magnetic decreases are caused by a diamagnetic effect from perpendicularly accelerated (to the magnetic field) protons. The ion acceleration is associated with the dissipation of phase-steepened Alfvén waves, presumably through the Ponderomotive Force. Proton perpendicular heating, through instabilities, lead to the generation of both proton cyclotron waves and mirror mode structures. Electromagnetic and electrostatic electron waves are detected as well. The Alfvén waves are thus found to be both dispersive and dissipative, conditions indicting that they may be intermediate shocks. The resultant "turbulence" created by the Alfvén wave dissipation is quite complex. There are both propagating (waves) and nonpropagating (mirror mode structures and MDs) byproducts. Arguments are presented to indicate that similar processes associated with Alfvén waves are occurring in the magnetosphere. In the magnetosphere, the "turbulence" is even further complicated by the damping of obliquely propagating proton cyclotron waves and the formation of electron holes, a form of solitary waves. Interplanetary Alfvén waves are shown to rapidly phase-steepen at a distance of 1AU from the Sun. A steepening rate of ~35 times per wavelength is indicated by Cluster-ACE measurements. Interplanetary (reverse) shock compression of Alfvén waves is noted to cause the rapid formation of MDs on the sunward side of corotating interaction regions (CIRs). Although much has been learned about the Alfvén wave phase-steepening processfrom space plasma observations, many facets are still not understood. Several of these topics are discussed for the interested researcher. Computer simulations and theoretical developments will be particularly useful in making further progress in this exciting new area.

Phase‐steepened Alfvén waves, proton perpendicular energization and the creation of magnetic holes and magnetic decreases: The ponderomotive force

Solar wind protons detected within Magnetic Holes (MHs) and Magnetic Decreases (MDs) are found to be preferentially heated perpendicular to 0. The MHs/MDs are associated with the phase‐steepened edges of nonlinear Alfvén waves. The proton anisotropies can lead to the proton cyclotron and mirror mode plasma instabilities. We examine the Ponderomotive Force (PF), a phenomenon due to wave pressure gradients, and show that for this plasma regime and for phase‐steepened Alfvén waves, the PF proton acceleration/energization will primarily be orthogonal to B0. It is suggested that accelerated ions create the MHs/MDs by a diamagnetic effect.

Relationship between discontinuities, magnetic holes, magnetic decreases, and nonlinear Alfvén waves: Ulysses observations over the solar poles

Ulysses magnetic field data are used to study magnetic field microstructure over the solar poles. Magnetic holes (MHs) and magnetic decreases (MDs) are found to be located at the phase‐steepened edges of nonlinear Alfvén waves. The phase‐steepened edges (directional discontinuities) occur in time‐coincidence with MHs, one edge of an MD, or throughout the whole MD. These MH‐ and MD‐related discontinuities have both rotational and compressive properties, perhaps explaining why many directional discontinuities detected in interplanetary space have non‐MHD properties. The dispersive, dissipative and compressive features of nonlinear Alfvén waves may be important for the heating of the solar corona.

Energetic particle cross-field diffusion: Interaction with Magnetic Decreases (MDs)

Abstract. Magnetic field Decreases (MDs) are detected in the heliospheric polar regions. The MDs have minimum spatial scales sizes of 25 proton thermal gyroradii, and are typically bounded by tangential or rotational discontinuities. The distribution of the magnitudes of the decreases within AIDs is a continuum, with the smallest decreases being most frequent in occurrence. The largest decreases can be 80% of the ambient field. The thickness distribution is also a continuum, and is shown to be independent of the field magnitude decrease. Charged particle interactions with the MDs lead to particle guiding center displacements and hence particle cross-held diffusion. We develop a diffusion model to apply to energetic ion interactions with MDs using the MD properties described in this paper. One specific day of data is used to illustrate that the particle cross-field diffusion will be extremely rapid due to such interactions.