Interplanetary Magnetic Field Strength Research Articles

Short large-amplitude magnetic structures (SLAMS) at Mercury observed by MESSENGER

Abstract. We present the first observations of short large-amplitude magnetic structures (denoted SLAMS) at Mercury. We have investigated approximately 4 years of MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) data to identify SLAMS in the Mercury foreshock. Defining SLAMS as magnetic field compressional structures, with an increase in magnetic field strength of at least twice the background magnetic field strength, when MESSENGER is located in the solar wind, we find 435 SLAMS. The SLAMS are found either in regions of a general ultra-low frequency (ULF) wave field, at the boundary of such a ULF wave field, or in a few cases isolated from the wave field. We present statistics on several properties of the SLAMS, such as temporal scale size, amplitude, and the presence of whistler-like wave emissions. We find that SLAMS are mostly found during periods of low interplanetary magnetic field strength, indicating that they are more common for higher solar wind Alfvénic Mach number (MA). We use the Tao solar wind model to estimate solar wind parameters to verify that MA is indeed larger during SLAMS observations than otherwise. Finally, we also investigate how SLAMS observations are related to foreshock geometry.

The Radial Interplanetary Field Strength at Sunspot Minimum as Polar Field Proxy and Solar Cycle Predictor

The minimum value of the geomagnetic aa index has served as a remarkably successful predictor of solar cycle amplitude. This value is reached near or just after sunspot minimum, when both the near-Earth solar wind speed and interplanetary magnetic field (IMF) strength fall to their lowest values. At this time, the heliospheric current sheet is flattened toward the heliographic equator and the dominant source of the IMF is the Sun’s axial dipole moment, which, in turn, has its source in the polar fields. As recognized previously, the success of aamin as solar cycle precursor provides support for dynamo models in which the sunspots of a given cycle are produced by winding up the poloidal field built up during the previous cycle. Because they are highly concentrated toward the poles by the surface meridional flow, the polar fields are difficult to measure reliably. Here we point out that the observed value of the radial IMF strength at solar minimum can be used to constrain the polar field measurements, and that this parameter, which is directly proportional to the Sun’s axial dipole strength, may be an even better solar cycle predictor than geomagnetic activity.

Mercury’s Bow Shock and Magnetopause Variations According to MESSENGER Data

Using data from the MESSENGER spacecraft magnetometer that describes the magnetopause and the bow shock crossing points of the Mercury’s magnetosphere, we have calculated the parameters of the paraboloids of revolution approximating the obtained points. For each spacecraft orbit, the subsolar magnetopause and bow shock standoff distances were obtained, based on the paraboloid parameters for each crossing point. The dependences of the magnetopause and bow shock subsolar standoff distances on the Mercury’s position relative to the Sun have been obtained. These profiles agree with decreases of the solar wind plasma dynamic pressure and the interplanetary magnetic field strength with heliocentric distance. The variations of the interplanetary and magnetosheath magnetic field were investigated. The average subsolar magnetosheath thickness and the value of the magnetic field jump at the bow shock during the transition from the upstream interplanetary magnetic field region to the magnetosheath were obtained.

The Influence of Solar Irradiation and Solar Wind Conditions on Heavy Ion Escape from Mars

AbstractWe apply a recently proposed method to estimate heavy ion escape from Mars. The method combines in situ observations with a hybrid plasma model, which treats ions as particles and electrons as a fluid. With this method, we investigate how solar upstream conditions, including solar extreme ultraviolet (EUV) radiation, solar wind dynamic pressure, and interplanetary magnetic field (IMF) strength and cone angle, affect the heavy ion loss. The results indicate that the heavy ion escape rate is greater in high EUV conditions. The escape rate increases with increasing solar wind dynamic pressure, and decreases as the IMF strength increases. The ion escape rate is highest when the solar wind is parallel to the IMF and lowest when they are perpendicular. The plume escape rate decreases when the solar wind convective electric field increases.

Solar-Cycle Variations of Forbidden Energetic Electrons Enhancements

Intense fluxes of electrons from the Earth’s radiation belt (ERB) with energies of tens and hundreds of keV can penetrate to low altitudes at low latitudes outside the South Atlantic Anomaly. This region is known as a forbidden zone of quasi-trapped energetic particles. Flux enhancements of energetic electrons in the forbidden zone, so-called forbidden energetic electrons (FEE), produce significant ionization effects in the upper atmosphere at low latitudes. In this work, solar-cycle variations of the FEE enhancements with energy > 30 keV were analyzed over a 25-year period using a database of low-orbit satellites of the NOAA/POES and MetOp series. We found the highest correlations of the annual occurrence of FEE with the F10.7 solar activity index (−0.87) and the Alfven Mach number of the upstream solar wind (0.76). Using multiparameter regression analysis, a power expression was obtained with those parameters as well as with plasma beta and the interplanetary magnetic field strength with a total correlation coefficient of 0.94. The role of the conductivity of the high-latitude ionosphere in the mechanism of the penetration of ERB electrons into the forbidden zone is discussed.

On the Relationship between the Fractal Dimension of Geomagnetic Variations at Altay and the Space Weather Characteristics

The fractal dimension of geomagnetic field component variations (horizontal—H, vertical—Z and magnetic declination—D) at the Baigazan magnetic station at Russian Altay, for the period 2011–2013, were calculated using the Higuchi method. The daily variation of Higuchi Fractal Dimension (HFD) for the D, H, Z components of the geomagnetic field were investigated, and its contribution to the variability of HFD was found to be from 30 to 40 percent of the total variance. A correlation analysis of the fractal dimension of the variations of the D, H, Z components with the Auroral Electrojet (AE) index and solar wind characteristics was carried out. Negative correlations with logarithms of the AE-index, interplanetary magnetic field (IMF) strength and solar wind velocity were found. About 25 percent of the HFD variance is controlled by the variability of these characteristics. Pair and partial correlation coefficients for these parameters were calculated for every month of 2011–2013.

Quantifying the Key Factors Affecting the Escape of Planetary Atmospheres

The habitability of Earth-like planets is an increasingly important subject in astrophysics and in planetary sciences. Atmospheric escape plays a vital role in the evolution of the habitability of Earth-like planets. By systematically analyzing the numerical simulation results of the interactions between the planetary atmospheres and the stellar winds, in this work, we evaluate various factors related to the atmospheric nonthermal ion escape rates, including planetary parameters (e.g., mass, density, radius, semimajor axis, etc.) and stellar wind parameters (e.g., density, velocity, and interplanetary magnetic field (IMF) strength). Furthermore, we determine and quantify the key factors affecting the planetary atmospheric nonthermal ion escape rates. Our results show that the correlation coefficients between planetary atmospheric nonthermal ion escape rates and stellar wind density, IMF strength, and the ratio of the planetary radius to the planetary semimajor axis are 0.98 (0.88), 0.95 (0.81), and 0.87 (0.59), respectively, in the scenario of maximum (minimum) dynamic wind pressure. This means that the planetary atmospheric nonthermal ion escape rates increase with the increasing stellar wind density, the increasing IMF strength, and the increasing ratio of the planetary radius to the planetary semimajor axis. Generally, the nonthermal ion escape rates of planetary atmospheres are more sensitive to stellar wind parameters than to others. In addition, we determine the functional relations of the above three significant parameters for evaluating and quantifying the effects of such key physical factors on the nonthermal ion escape rates of the planetary atmospheres. Our findings will be very useful for better understanding the key factors that influence the escapes of planetary atmospheres.

Which Component of Solar Magnetic Field Drives the Evolution of Interplanetary Magnetic Field over the Solar Cycle?

The solar magnetic structure changes over the solar cycle. It has a dipole structure during solar minimum, where the open flux extends mainly from the polar regions into the interplanetary space. During maximum, a complex structure is formed with low-latitude active regions and weakened polar fields, resulting in spread open field regions. However, the components of the solar magnetic field that are responsible for long-term variations in the interplanetary magnetic field (IMF) are not clear, and the IMF strength estimated based on the solar magnetic field is known to be underestimated by a factor of 3–4 against the actual in situ observations (the open flux problem). To this end, we decomposed the coronal magnetic field into the components of the spherical harmonic function of degree and order (ℓ, m) using the potential field source surface model with synoptic maps from SDO/HMI for 2010–2021. As a result, we found that the IMF rapidly increased in 2014 December (7 months after the solar maximum), which coincided with the increase in the equatorial dipole, (ℓ, m) = (1, ±1), corresponding to the diffusion of active regions toward the poles and in the longitudinal direction. The IMF gradually decreased until 2019 December (solar minimum) and its variation corresponded to that of the nondipole component ℓ ≥ 2. Our results suggest that the understanding of the open flux problem may be improved by focusing on the equatorial dipole and the nondipole component and that the influence of the polar magnetic field is less significant.

Transient Foreshock Structures Upstream of Mars: Implications of the Small Martian Bow Shock

AbstractThe typical subsolar stand‐off distance of Mars' bow shock is of the order of a solar wind ion convective gyroradius, making it highly non‐planar to incident ions. Using spacecraft observations and a test particle model, we illustrate the impact of the bow shock curvature on transient structures which form near the upstream edge of moving foreshocks caused by slow rotations in the interplanetary magnetic field (IMF). The structures exhibit noticeable decrease in the solar wind plasma density and the IMF strength within their core, are accompanied by a compressional shock layer, and are consistent with foreshock bubbles (FBs). Ion populations responsible for these structures include backstreaming ions that only appear within the moving foreshock and reflected ions with hybrid trajectories that straddle between the quasi‐perpendicular and quasi‐parallel bow shocks during slow IMF rotations. Both ion populations accumulate near the upstream edge of the moving foreshock which facilitates FB formation.

Statistical comparison of time profiles of Forbush decreases associated with coronal mass ejections and streams from coronal holes in solar cycles 23–24

ABSTRACTIn this paper, Forbush decrease (FD) profiles are compared for events associated with (i) coronal mass ejections from active regions accompanied by solar flares (AR CMEs), (ii) filament eruptions away from active regions (non-AR CMEs), and (iii) high-speed streams (HSSs) from coronal holes (CHs). FD profiles are described by time parameters that are delayed from an FD onset to the registration of maximum values of cosmic ray (CR) density variations, CR density hourly decrease, CR equatorial anisotropy, solar wind (SW) speed, interplanetary magnetic field (IMF) strength and minimum Dst index. Distributions of these parameters from 1997 to 2020 and within maxima and minima of the last solar cycles (SCs) were compared by statistical methods. The results obtained reveal that statistical properties of the time parameters depend both on the FD source and on the solar activity period. FDs associated with AR CMEs develop even at close values of SW parameters faster than those associated with non-AR CMEs and HSS from CHs. Differences between typical FD profiles for events associated with AR and non-AR CMEs are more significant when the interplanetary disturbance contains a magnetic cloud. The difference between FD profiles for events associated with AR and non-AR CMEs is less distinguishable within maximum SC 24 than within maximum SC 23. For FDs associated with HSS from CHs, the main phase durations and the time delays of maximal SW speed are longer within SC 23–24 minimum, while the time delays of maximal IMF strength differ insignificantly between 23–24 and 24–25 minima.

The Dawn–Dusk Asymmetrical Distribution of Earthward Poynting Flux in the Dayside Polar Cap From DMSP

AbstractWe investigated the dawn–dusk asymmetrical distribution of quasi‐dc earthward Poynting flux (EPF) in the dayside polar cap of the Northern Hemisphere based on 3 years of observations at ∼850 km made by the Defense Meteorological Satellite Program (DMSP) F17. The satellite data have been sorted by low (Kp ≤ 2) and high (Kp > 2) geomagnetic activity, as well as the orientation and strength of interplanetary magnetic field (IMF) By and Bz components, binned by magnetic latitude, and magnetic local time. Statistical results show that the enhanced convection electric field favors the asymmetrical EPF distribution. Asymmetrical distribution characteristics are summarized as (a) there are mainly two enhanced EPF regions, near dawn and cusp and the mean EPF is 28% larger on the dawn than on the dusk for all IMF and Kp conditions. (b) The mean EPF for high Kp is 3 times greater than that for low Kp in the dayside polar cap. The enhanced EPF shifted from near the dawn for low Kp to near the cusp for high Kp. The EPF is also larger on the dawn than on the dusk for high Kp. (c) The EPF of the dayside polar cap is significantly enhanced as the IMF strength increases. Under IMF By+, EPF is greater on the dawn than on the dusk. The opposite is true under IMF By−. The IMF Bz− favors enhanced EPF and the EPF on the dawn (dusk) reaches its maximum for IMF By+ and Bz− (IMF By− and Bz−).

Proton Precipitation in Mercury's Northern Magnetospheric Cusp

AbstractIon precipitation onto Mercury's surface through its magnetospheric cusps acts as a source of planetary atoms to both Mercury's exosphere and magnetosphere. Through the process of ion sputtering, solar wind ions (∼95% protons) impact the surface regolith and liberate material, mostly as neutral atoms. We have identified 2760 northern magnetospheric cusp crossings throughout the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission, based on enhancements in proton flux observed by the Fast Imaging Plasma Spectrometer (FIPS). We find cusp crossings spanning 50–85° in magnetic latitude with a geometric center typically at 60–70°. The cusp center is stable about its average but its latitudinal extent varies orbit‐to‐orbit. The mean latitude weakly depends on the magnitude of the interplanetary magnetic field (IMF), dropping by about 1.3° magnetic latitude for each increase of 10 nT in IMF strength. We have used these identified cusp boundaries to estimate the flux of protons which will precipitate onto Mercury's surface. We find an average proton precipitation flux of 1.0 × 107 cm−2 s−1, ranging 3.3 × 104–6.2 × 108 cm−2 s−1, and that this flux can vary substantially between subsequent 10‐s measurements. We also tabulated the peak precipitation fluxes for each cusp crossing. They range 9.8 × 104–1.4 × 109 cm−2 s−1, with a mean of 3.7 × 107 cm−2 s−1. We find strong dependencies on the local time of the cusp crossing as well as on Mercury's orbit around the Sun, which warrant further investigation.

Statistical Plasma Properties of the Planar and Nonplanar ICME Magnetic Clouds during Solar Cycles 23 and 24

Various remote and in situ observations, along with several models, simulations, and kinetic studies, have been proposed in recent years, suggesting that the morphology of an interplanetary coronal mass ejection (ICME) magnetic cloud can vary from cylindrical, elliptical, toroidal, flattened, pancaked, etc. Recently, Raghav et al. proposed for the first time a unique morphological characteristic of an ICME magnetic cloud at 1 au that showed characteristics of a planar magnetic structure, using in situ data from the ACE spacecraft. In this study, we statistically investigate the plasma properties of planar and nonplanar ICMEs from 1998–2017 at 1 au. The detailed study of 469 ICMEs suggests that 136 (∼29%) ICMEs are planar, whereas 333 (∼71%) are nonplanar. Furthermore, total interplanetary magnetic field strength, average plasma parameters, i.e., plasma density, beta, thermal pressure, and magnetic pressure in planar ICME, are significantly higher than in the nonplanar ICME. Also, we noticed that the thickness of planar ICMEs is less compared to nonplanar ICMEs. This analysis demonstrates that planar ICMEs are formed due to the high compression of ICME. Moreover, we also observed the southward/northward magnetic field component’s double strength during planar ICMEs compared to nonplanar ICMEs. It implies that planar ICMEs are more geoeffective than nonplanar ICMEs.

A global view of Pc3 wave activity in near-Earth space: Results from hybrid-Vlasov simulations

Ultra-low frequency (ULF) waves in the Pc3 range, with periods between 10–45 s, are routinely observed in Earth’s dayside magnetosphere. They are thought to originate in the foreshock, which extends upstream of the quasi-parallel bow shock and is populated with shock-reflected particles. The foreshock is permeated with ULF waves generated by ion beam instabilities, most notably the “30-s” waves whose periods match those of the Pc3 waves and which are carried earthward by the solar wind flow. However, the global picture of Pc3 wave activity from the foreshock to the magnetosphere and its response to changing solar wind conditions is still poorly understood. In this study, we investigate the global distribution and properties of Pc3 waves across near-Earth space using global simulations performed with the hybrid-Vlasov model Vlasiator. The simulations enable us to study the waves in their global context, and compare their properties in the foreshock, magnetosheath and dayside magnetosphere, for different sets of upstream solar wind conditions. We find that in all three regions the Pc3 wave power peaks at higher frequencies when the interplanetary magnetic field (IMF) strength is larger, consistent with previous studies. The Pc3 wave power is significantly enhanced in all three regions for higher solar wind Alfvén Mach number. As this parameter is known to affect the shock properties but has little impact inside the magnetosphere, this brings further support to the magnetospheric waves originating in the foreshock. Other parameters that are found to influence the foreshock wave power are the solar wind density and the IMF cone angle. Inside the magnetosphere, the wave power distribution depends strongly on the IMF orientation, which controls the foreshock position upstream of the bow shock. The wave power is largest when the angle between the IMF and the Sun-Earth line is smallest, suggesting that wave generation and transmission are most efficient in these conditions.

Solar and Geomagnetic Activity Enhance the Effects of Air Pollutants on Atrial Fibrillation

Cardiac arrhythmias have been associated with intense solar and geomagnetic activity (SGA) and exposures to air pollution.We examined whether oscillations of SGA can modify the effect of hourly exposures to air pollutants on atrial fibrillation ≥30 seconds (AF) risk in patients with dual chamber implantable cardioverter defibrillators (ICD). The effects of SGA on ambient particulate matter <2.5 µm (PM2.5), black carbon (BC), ultrafine particles (PN), and associations with AF were assessed. Measures of SGA included solar wind proton density (SW), total interplanetary magnetic field strength (IMF), and Kp index, a measure of global geomagnetic activity. Over all time lags between 0 and 24hr, periods of increased SGA (>50th percentile in SGA) enhanced the effects of all three air pollutants on AF, while during periods of reduced SGA the associations were considerably weaker or absent. During periods of intense SW 6 hours prior to an AF event, the odds ratio (OR) for PM2.5 exposure per IQR of 5.6 µg/m3 was 1.7 (95% CI: 1.3 to 2.3, p-value: 0.0001). For periods of reduced SW, the OR for PM2.5 exposure per IQR was 1.2 (95% CI: 0.9 to 1.5; p-value: 0.27). There were similar effects for PN and BC exposures. In patients with multiple AF events per hour, the associations with air pollutants during intense SGA were even greater. The effects of air pollutants up to 24 h before AF events were enhanced during periods of increased SGA. Our results suggest that these effects may account for variation in AF risk.

Improving Multiday Solar Wind Speed Forecasts

AbstractWe analyze the residual errors for the Wang‐Sheeley‐Arge (WSA) solar wind speed forecasts as a function of the photospheric magnetic field expansion factor (fp) and the minimum separation angle (d) in the photosphere between the footpoints of open field lines and the nearest coronal hole boundary. We find the map of residual speed errors are systematic when examined as a function of fp and d. We use these residual error maps to apply corrections to the model speeds. We test this correction approach using 3‐day lead time speed forecasts for an entire year of observations and model results. Our methods can readily be applied to develop corrections for the remaining WSA forecast lead times which range from 1 to 7 days in 1‐day increments. Since the solar wind density, temperature, and the interplanetary magnetic field strength all correlate well with the solar wind speed, the improved accuracy of solar wind speed forecasts enables the production of multiday forecasts of the solar wind density, temperature, pressure, and interplanetary field strength, and geophysical indices. These additional parameters would expand the usefulness of Air Force Data Assimilative Photospheric Flux Transport‐WSA forecasts for space weather clients.

Nightside Auroral Electrons at Mars: Upstream Drivers and Ionospheric Impact

AbstractDiscrete aurorae have been observed at Mars by multiple spacecraft, including Mars Express, Mars Atmosphere and Volatile EvolutioN (MAVEN), and most recently the United Arab Emirates Hope spacecraft. Meanwhile, there have been studies on the source particles responsible for producing these detectable aurorae (termed “auroral electrons”). By utilizing empirical criteria to select auroral electrons established by Xu et al. (2022, https://doi.org/10.1029/2022GL097757), we conduct statistical analyses of the impact of upstream drivers on the occurrence rate and fluxes of auroral electrons. We find the occurrence rate increases with upstream dynamic pressure and weakly depends on the interplanetary magnetic field strength. Meanwhile, the integrated auroral electron flux somewhat decreases with increasing upstream drivers. Auroral electron precipitation also occurs more frequently and is more intense over regions of strong crustal fields compared to weak crustal fields. Aside from emissions, auroral electrons are expected to cause significant impact ionization and enhance the plasma density locally. In this study, we also quantify the nightside ionospheric impact of auroral electron precipitation, specifically the thermal ion (O+, , and ) density enhancement, with MAVEN observations. Our results show that the ion density is increased by up to an order of magnitude at low altitudes. The crustal effects on ion density profiles for nominal electron and auroral electron precipitation are also discussed.

Electron-scale Magnetic Peaks Upstream of Mercury’s Bow Shock: MESSENGER Observations

Electron-scale magnetic peaks (ESMPs) with spatial sizes less than one local ion gyroradius have been recently revealed to exist in the terrestrial magnetosheath and solar wind at 1 au. Whether they widely exist in the astrophysical plasma is unclear. Here, we investigate the magnetic peaks with a period of 0.1–100 s upstream of Mercury’s bow shock by using the magnetic field data from the MESSENGER spacecraft. Based on the distribution of their durations, these magnetic peaks can be divided into two groups: one with durations less than 0.6 s and the other with durations larger than 0.6 s. The durations in each group obey a log-normal distribution. The magnetic peaks with durations less than 0.6 s are inferred to be electron scale, suggesting that ESMPs exist in the solar wind at Mercury’s orbit. The median duration of these ESMPs is ∼0.3 s. The ESMPs have a larger occurrence rate near the bow shock and prefer to occur when the ambient interplanetary magnetic field (IMF) can be connected to the bow shock, which suggests that the foreshock could be one source region of these ESMPs. Their occurrence rate also tends to be larger when the IMF strength is weaker. Our observations also suggest that some ESMPs originate from the upstream solar wind. The properties of the ESMPs found here could help to shed light on their generation mechanisms and their roles in the astrophysical plasma.

A New Method of Fluxgate Magnetometer Offset Vector Determination in the Solar Wind Using Any Magnetic Field Variations

In-depth study of dynamic processes in the astrophysical plasma environment relies on high-precision measurement of the magnetic field. Fluxgate magnetometers (FGMs) are commonly used on spacecraft to measure the magnetic field. However, their zero offsets vary slowly with time, and therefore need regularly in-flight calibration. Traditional methods of calculating the zero offset are based on properties of Alfvén waves, mirror mode structures, or current sheets. Here, we develop a new method of calculating the zero offset using any interplanetary magnetic field (IMF) variations. We create an offset cube according to the possible range of the IMF strength. The average values of B L for the IMF variation events approximately obey the normal distribution if there are enough events, where B L is the magnetic field in the maximum variance direction. Any constant vector added to the natural magnetic field data of the events will make the standard deviation of the normal distribution larger. Thereby, the point is determined to be the zero offset so that the corresponding standard deviation at this point is the minimum in the offset cube. Our test results show that this method has a 95.5% probability of obtaining the zero offset with an error of less than 0.3 nT when 10–21 hr of data are used. Our method provides an option for the in-flight calibration of the spaceborne FGM in the solar wind when there are not enough Alfvén waves, mirror modes, or current sheets.

Forbush decreases associated with coronal mass ejections from active and non-active regions: statistical comparison

ABSTRACT In this paper, Forbush decreases (FDs) from 1997 to 2020 associated with coronal mass ejections from active and non-active regions are compared between themselves and to FDs caused by high-speed streams from coronal holes. The two types of sporadic FDs are also compared when corresponding solar wind (SW) disturbances contain, or do not contain, magnetic clouds (MCs) near Earth. Cosmic ray density and anisotropy variations, SW speed, interplanetary magnetic field (IMF) strength, and geomagnetic indices have been examined using statistical methods. The results reveal that these parameters are larger for FDs associated with active region (AR) ejections and have highly skewed distributions for both types of sporadic events. In the same ranges of SW parameters, FD magnitude is larger for flare-associated events; more efficient modulation occurs in FDs associated with AR ejections. Differences between FDs associated with AR and non-AR ejections are more pronounced when an MC is registered. For IMF strength and geomagnetic indices, differences between the distributions depend more upon MC presence or absence than on the type of solar source. Correlation of IMF strength and SW speed differs slightly between FDs caused by AR and non-AR ejections regardless of the presence or absence of an MC, akin to the partial correlation between FD magnitude and IMF strength. Difference between the speeds of disturbed and background SW is larger for FDs associated with AR ejections especially when an MC is registered; the interaction region of different-speed SW streams occurs more frequently in interplanetary disturbances induced by AR ejections.