Solar Wind Magnetic Field Research Articles

Classifier Neural Network Models Predict Relativistic Electron Events at Geosynchronous Orbit Better than Multiple Regression or ARMAX Models

AbstractTo find the best method of predicting when daily relativistic electron flux (>2 MeV) will rise at geosynchronous orbit, we compare model predictive success rates (true positive rate or TPR) for multiple regression, ARMAX, logistic regression, a feed‐forward multilayer perceptron (MLP), and a recurrent neural network (RNN) model. We use only those days on which flux could rise, removing days when flux is already high from the data set. We explore three input variable sets: (1) ground‐based data (Kp, Dst, and sunspot number), (2) a full set of easily available solar wind and interplanetary magnetic field parameters (|B|, Bz, V, N, P, Ey, Kp, Dst, and sunspot number, and (3) this full set with the addition of previous day's flux. Despite high validation correlations in the multiple regression and ARMAX predictions, these regression models had low predictive ability (TPR < 45%) and are not recommended for use. The three classifier model types (logistic regression, MLP, and RNN) performed better (TPR: 50.8–74.6%). These rates were increased further if the cost of missing an event was set at 4 times that of predicting an event that did not happen (TPR: 73.1–89.6%). The area under the receiver operating characteristic curves did not, for the most part, differ between the classifier models (logistic, MLP, and RNN), indicating that any of the three could be used to discriminate between events and nonevents, but validation suggests a full RNN model performs best. The addition of previous day's flux as a predictor provided only a slight advantage.

Flux Erosion of Magnetic Clouds by Reconnection With the Sun's Open Flux

AbstractMagnetic clouds (MCs) are flux rope magnetic structures forming a subset of solar coronal mass ejections, which have significant space weather impacts. The geoeffectiveness of MCs depends on their properties, which evolve during their interplanetary passage. Based on an analysis of observations spanning two solar cycles, we establish that MCs interacting with the ambient solar wind magnetic field (i.e., heliospheric open flux) lose a substantial amount of their initial magnetic flux via magnetic reconnection, which, in some cases, reduce their geoeffectiveness. We find a linear correlation between the eroded flux of MCs and solar open flux which is consistent with the scenario that MC erosion is mediated via the local heliospheric magnetic field draping around an MC during its interplanetary propagation. The solar open flux is governed by the sunspot cycle. This work therefore uncovers a hitherto unknown pathway for solar cycle modulation of the properties of MCs.

Study of the fractality in a magnetohydrodynamic shell model forced by solar wind fluctuations

Abstract. The description of the relationship between interplanetary plasma and geomagnetic activity requires complex models. Drastically reducing the ambition of describing this detailed complex interaction and, if we are interested only in the fractality properties of the time series of its characteristic parameters, a magnetohydrodynamic (MHD) shell model forced using solar wind data might provide a possible novel approach. In this paper we study the relation between the activity of the magnetic energy dissipation rate obtained in one such model, which may describe geomagnetic activity, and the fractal dimension of the forcing. In different shell model simulations, the forcing is provided by the solution of a Langevin equation where a white noise is implemented. This forcing, however, has been shown to be unsuitable for describing the solar wind action on the model. Thus, we propose to consider the fluctuations of the product between the velocity and the magnetic field solar wind data as the noise in the Langevin equation, the solution of which provides the forcing in the magnetic field equation. We compare the fractal dimension of the magnetic energy dissipation rate obtained, of the magnetic forcing term, and of the fluctuations of v⋅bz, with the activity of the magnetic energy dissipation rate. We examine the dependence of these fractal dimensions on the solar cycle. We show that all measures of activity have a peak near solar maximum. Moreover, both the fractal dimension computed for the fluctuations of v⋅bz time series and the fractal dimension of the magnetic forcing have a minimum near solar maximum. This suggests that the complexity of the noise term in the Langevin equation may have a strong effect on the activity of the magnetic energy dissipation rate.

Solitons and Turbulence in Solar Wind at 1 au: Multi-satellite Data and FDTD Simulations

Abstract During 2015 January 1–31 multi-satellite data at 1 au showed the propagation of single soliton pulses, the formation of soliton trains, and their ultimate development into the turbulence in solar wind magnetic field, density, speed, temperature, and dynamic pressure. This work is motivated by a basic research question: what are the differences in the conditions for excitation of soliton pulse, soliton train, and turbulence in the solar wind? To answer this question, a convective and dispersive system is considered and simulated via the finite difference time domain (FDTD) method. It is found that when a system is initially driven by a weak shock wave only a single soliton pulse will propagate. When the convection coefficient is smaller than or equal to the dispersion coefficient and system is driven by a strong shock then soliton trains are formed. The development of turbulence occurs when the convective coefficient dominates the dispersion coefficient and the system is driven by a stronger shock. Combining the findings of FDTD simulations with the Hall magnetohydrodynamic model, it is concluded that an enhanced interplanetary magnetic field in the magnetic sheath and the declining ion density after corotation interaction region interface provide favorable conditions for the solitons formation. Contrarily, the declining magnetic field in ejecta provides pertinent conditions for the evolution of Alfvénic turbulence. Our findings are critical for understanding the wave development into turbulence in solar wind.

Plasma Waves near the Electron Cyclotron Frequency in the Near-Sun Solar Wind

Data from the first two orbits of the Sun by Parker Solar Probe reveal that the solar wind sunward of 50 solar radii is replete with plasma waves and instabilities. One of the most prominent plasma wave power enhancements in this region appears near the electron cyclotron frequency (f_ce). Most of this wave power is concentrated in electric field fluctuations near 0.7 f_ce and f_ce, with strong harmonics of both frequencies extending above f_ce. At least two distinct, often concurrent, wave modes are observed, preliminarily identified as electrostatic whistler-mode waves and electron Bernstein waves. Wave intervals range in duration from a few seconds to hours. Both the amplitudes and number of detections of these near-f_ce waves increase significantly with decreasing distance to the Sun, suggesting that they play an important role in the evolution of electron populations in the near-Sun solar wind. Correlations are found between the detection of these waves and properties of solar wind electron populations, including electron core drift, implying that these waves play a role in regulating the heat flux carried by solar wind electrons. Observation of these near-f_ce waves is found to be strongly correlated with near-radial solar wind magnetic field configurations with low levels of magnetic turbulence. A scenario for the growth of these waves is presented which implies that regions of low-turbulence near-radial magnetic field are a prominent feature of solar wind structure near the Sun.

Solar Wind and Interplanetary Magnetic Field Influence on Ultralow Frequency Waves and Reflected Ions Near the Moon.

With no global magnetic field or atmosphere, the Moon was traditionally seen as a perfect absorber of the incoming solar wind. Recently, it has become apparent that magnetic fields with sources in the lunar crust act to reflect a significant percentage of incoming solar wind particles, which can then interact with the surrounding plasma environment and drive plasma waves. Using data collected by the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft, we look for simultaneous observations of reflected ions and 0.01 Hz waves to study the characteristics and conditions under which wave-particle resonant interactions occur. Analyzing the solar wind and interplanetary magnetic field during these observations reveals particular solar wind and interplanetary magnetic field conditions that favor the generation of these waves. We use an ion tracing program to produce reflected ion distributions for various ambient conditions. These distributions show that the conditions that lead to more ions crossing the equatorial region where ARTEMIS orbits are also those favored for wave observations. Low-frequency waves, such as those generated by cyclotron resonance with ions, can be heavily Doppler shifted, making it difficult to determine their intrinsic properties. Reflected ion distributions for the same ambient conditions as the observed waves suggest that most of the waves are intrinsically right-hand polarized.

The Solar Wind Speed Expansion Factor [${v}$–$f _{\text{s}}$] Relationship at the Inner Boundary (18 $\text{R}_{\odot }$) of the Heliosphere

The accuracy of data-driven magnetohydrodynamics (MHD) models depends on accurate boundary conditions specified at the inner heliosphere. However, not all of the MHD parameters [$\boldsymbol{B}, \boldsymbol{v}, \rho , T$] are measurable close to the Sun at the present time, except the vector magnetic field [$\boldsymbol{B} $] at the photosphere. The solar wind speed [$\boldsymbol{v}$], which is probably most relevant to space-weather forecasting, is often modeled by the standard Wang–Sheeley (WS) formula, which is based on an inverse relationship between the solar wind speed [$\boldsymbol{v}$] at 1 AU and the expansion factor [$f_{\text{s}}$] estimated at 2.5 solar radii [$\text{R}_{ \odot }$], with the following generic form: ${v} = {v}_{1} + {v}_{2} f_{\text{s}}^{- \alpha }$ (where $\boldsymbol{v}$ is the solar wind speed at 18 $\text{R}_{\odot }$, $f_{\text{s}}$ is the magnetic-field expansion factor, and ${v}_{1}$, ${v}_{2}$, and $\alpha $ are three free parameters to be determined). While the WS formula uses “source projection” to determine the solar wind source, it does not treat the solar wind as plasma because it uses the solar wind speed observed at 1 AU to derive the empirical relationship. Thus, the resulting formula ignores the transport and acceleration of the solar wind as it propagates out into the heliosphere. The purpose of this study is to rectify this omission by using a numerical MHD simulation to find the optimal set of free parameters that relate the magnetic properties at the source surface to the plasma parameters at 1 AU. In addition to the expansion factor, conservation of mass [$\rho \boldsymbol{v}$], magnetic flux [$r ^{2} B$], and total pressure along the stream line are assumed to obtain the solar wind mass density, magnetic field, and temperature at 18 $\text{R}_{\odot }$. These parameters are used as the inner boundary conditions of our global three-dimensional MHD (G3DMHD) code to simulate solar wind plasma and field parameters out to ${\approx}\, 1~\mbox{AU}$. The simulation results are compared with the in-situ data from Wind to assess the accuracy. Such a procedure is repeated (880 times) to cover the three parameter regimes ($100 < {v}_{1} < 350~\mbox{km}\,\mbox{s}^{-1}$; $250 < {v}_{2} < 700~\mbox{km}\,\mbox{s}^{-1}$; and $0.2 < \alpha < 0.9$) to find the optimal set. The simulation is performed for the period of CR2082 [30 March 2019 to 27 April 2009]. It is found that $\boldsymbol{v} = 189 + 679 f_{\text{s}}^{-0.7}$ is the best formula to relate the solar wind speed at 18 $\text{R}_{\odot }$ to the expansion factor. Strictly speaking, this formula is most applicable for solar equatorial regions and near the times of solar minimum when there are few coronal mass ejection events.

Neural Network Recovery of Gaps in Geomagnetic Field Records

The paper demonstrates the capabilities of neural network recovery of ground-based geomagnetic field records at a selected magnetic station using similar magnetic field data at another station. By the example of the restoration of disturbance records made at the magnetic stations Kakioka, Kanoya, Alma-Ata, Hermanius, San Juan, Tucson, Honolulu with and without data from the OMNI satellite system on the parameters of the solar wind and interplanetary magnetic field, it is shown that the technique of artificial neural networks can to successfully fill in the gaps and failures in the records of individual observatories of the global network of magnetic observation stations. The created artificial neural network tool can be used for scientific and applied problems of geomagnetic information recovery.

Magnetic Field Variations and Dynamics of the Outer Electron Radiation Belt of the Earth’s Magnetosphere in February 2014

The paper presents the results of a comparative analysis of the dynamics of relativistic electron fluxes of the Earth’s outer radiation belt and correspondent ring current and magnetospheric magnetic field variations for February 15–22, 2014, based on experimental data obtained in the radiation belt core aboard Van Allen Probes satellites and in geostationary orbit aboard the GOES-15 satellite. The A2000 paraboloid model of the magnetosphere is used to calculate the contributions of main magnetospheric current systems to the Dst variation. The influence of external factors—solar wind and interplanetary magnetic field—is considered. Comparison of the dynamics of electron fluxes in different orbits and the geomagnetic field variations for February 15‒22, 2014 indicate that the main mechanisms of the evolution of outer electron radiation belt are global processes: (i) outward and inward shifts of the trapped electron population inside the Earth’s magnetosphere due to large-scale variations of the magnetospheric magnetic field during geomagnetic disturbances and (ii) ExB drift of electrons from the magnetotail under the influence of electric and magnetic fields. Local particle acceleration is auxiliary process in the dynamics of radiation belts. The southward orientation of the interplanetary magnetic field is the necessary condition for an increase in the intensity of electron fluxes of the Earth’s outer radiation belt, while the function (–BzVsw) is the key external factor.

Ultra-low-frequency waves in the ion foreshock of Mercury: a global hybrid modelling study

ABSTRACT We study the solar wind interaction with Mercury using a global three-dimensional hybrid model. In the analysed simulation run, we find a well-developed, dynamic Hermean ion foreshock ahead of the quasi-parallel bow shock under upstream solar wind and interplanetary magnetic field (IMF) conditions corresponding to the orbital perihelion of the planet. A portion of the incident solar wind ion flux is scattered back upstream near the quasi-parallel bow shock including both major solar wind ion species, protons and alphas. The scattered particles form the Hermean suprathermal foreshock ion population. A significant part of the suprathermal population is backstreaming with a velocity component towards the Sun in the near-foreshock at the planetocentric distance of few planetary radii in the plane of the IMF. The ion foreshock is associated with large-scale, oblique fast magnetosonic waves in the ultra-low-frequency (ULF) range convecting downstream with the solar wind. The ULF wave period is about 5 s in the analysed upstream condition case at Mercury, which corresponds to the 30-s foreshock waves at Earth when scaled by the IMF magnitude.

Magnetosphere response to stationary solar wind forcing at various plasma resistivities in 2D MHD simulation

The magnetosphere response to the stationary solar wind forcing at various values of the plasma resistivity is studied. The simulation is performed within two-dimensional (2D) MHD resistive model; the meridional plane is considered. The solar wind and southward interplanetary magnetic field (IMF) are supposed to be stationary. The calculations were carried out for two plasma resistivity values ηp = {106; 104} Ω·m, assumed constant in the whole flow field. At higher plasma resistivity the steady magnetosphere convection (SMC) takes place; the solution is symmetric relative to the x-axis. At lower resistivity the sawtooth event is realized; the solution is quasi-periodic and asymmetric. The asymmetry is connected with the formation of the vortex street of Karman type. The sequence of phases of the sawtooth cycle, as well as the evolution of magnetic lines of force and the flow structure in each phase is analyzed. In both solutions the day side magnetic reconnection occurs through the two x-points disposed symmetrically at the top and bottom peaks of the magnetic island.

Expansion and Shrinking of the Martian Topside Ionosphere

AbstractThe observations made by the Mars Atmosphere and Volatile EvolutioN spacecraft in the topside (≥200 km) ionosphere of Mars show that this region is very responsive to the variations of the external (solar extreme ultraviolet flux, solar wind, and interplanetary magnetic field [IMF]) and internal (the crustal magnetic field) drivers. With the growth of the solar irradiance the ionosphere broadens while with increase of the solar wind dynamic pressure it shrinks. As a result, the upper ionospheric boundary at solar zenith angles of 60–70° can move from ∼400 to ∼1,200 km. Similar trends are observed at the nightside ionosphere. At Pdyn ≥ 1–2 nPa the nightside ionosphere becomes very fragmented and depleted. On the other hand, the ion density in the nightside ionosphere significantly (up to a factor of 10) increases with the rise of the solar extreme ultraviolet flux. Large‐amplitude motions of the topside ionosphere also occur with variations of the value of the cross‐flow component of the IMF. The upper dayside ionosphere at altitudes of more than 300–400 km is sensitive also to the direction of the cross‐flow component of the IMF or, correspondingly, to the direction of the motional electric field in the solar wind. The ionosphere becomes very asymmetrical with respect to the Vsw×BIMF direction and the asymmetry strongly enhances at the nightside. The topside ionosphere above the areas with strong crustal magnetic field in the dayside southern hemisphere is significantly denser and expands to higher altitudes as compared to the ionosphere above the northern nonmagnetized lowlands. The crustal magnetic field also protects the nightside ionosphere from being filled by plasma transported from the dayside. The draping IMF penetrates deeply into the ionosphere and actively influences its structure. Weak fields and, correspondingly, weak magnetic field forces only slightly affect the ionosphere. With increase of the induced magnetic field strength the transport motions driven by the magnetic field pressure and field tensions seem to be intensified and we observe that the local ion densities at the dayside considerably decrease. A different trend is observed at the nightside. The ion density in the nightside ionosphere above the northern lowlands is higher than in the southern hemisphere indicating that plasma transport from the dayside is the main source of the nightside ionosphere. Nonstop variations in the solar wind, the IMF and the solar irradiance together with planetary rotation of the crustal magnetic field sources lead to a continuous expansion/shrinking and reconfiguration of the topside ionosphere of Mars.

Multiple-point Modeling the Parker Spiral Configuration of the Solar Wind Magnetic Field at the Solar Maximum of Solar Cycle 24

Abstract By assuming that the solar wind flow is spherically symmetric and that the flow speed becomes constant beyond some critical distance r = R 0 (neglecting solar gravitation and acceleration by high coronal temperature), the large-scale solar wind magnetic field lines are distorted into a Parker spiral configuration, which is usually simplified to an Archimedes spiral. Using magnetic field observations near Mercury, Venus, and Earth during solar maximum of Solar Cycle 24, we statistically surveyed the Parker spiral angles and obtained the empirical equations of the Archimedes and Parker spirals by fitting the multiple-point results. We found that the solar wind magnetic field configurations are slightly different during different years. Archimedes and Parker spiral configurations are quite different from each other within 1 au. Our results provide empirical Archimedes and Parker spiral equations that depend on the solar wind velocity and the critical distance (R 0). It is inferred that R 0 is much larger than that previously assumed. In the near future, the statistical survey of the near-Sun solar wind velocity by Parker Solar Probe can help verify this result.

Storm-time mesoscale field-aligned currents and interplanetary parameters

Present paper studies Field aligned currents (FACs) estimated by employing Ampere's law to the magnetic field recorded by CHAMP satellite during 24 geomagnetic storms. Low-pass filtered FACs with a cutoff period of 20 s (scale size~150 km) are used to determine FAC range, which is defined as a peak-to-peak amplitude of FAC density. Thus we are considering only the strongest positive and negative FACs emerging either from Region 1, Region 2, Region 0, or substorm current wedge systems. It is known that the FACs significantly depend on the highly variable solar wind (SW) and interplanetary magnetic field (IMF) conditions and also on the processes internal to magnetospheric-ionospheric system such as substorm. The correlation analysis carried out here shows that sometimes the FAC range, correlates well with SymH, AsyH, AsyD, AL, am and Kp indices (>95% significance), but not always. The variation of the FAC range with magnetic local times shows distinctly different patterns during southward and northward IMF conditions, with peaks near dawn-dusk during southward IMF and near local noon-midnight during northward IMF. These results are in agreement with the earlier reports. However, the seasonal dependence reveals that the noon time peak is essentially associated with the summer season. We have determined a new parameter called ‘occurrence rate of FAC range ≥1 μA/m2’ and examined it under various solar wind and IMF conditions. It is found that the probability of FAC range ≥1 μA/m2 have a clear dependence on the clock angle, suggesting more frequent intensifications during southward IMF. Clear linear dependence on the cone angle demonstrates higher occurrence probability of FAC range ≥ 1 μA/m2 when the IMF is perpendicular to the Sun-Earth line (cone angle nearing 90°). All these results based on the newly defined parameters such as FAC range and probability of FAC range ≥1 μA/m2, for the storm time mesoscale FAC are consistent with the previous studies. The FAC ranges are found to have a linear dependence on the values of IMF BY, BYZ, BT and BZ, though saturation is apparent at higher values of the IMF parameters. FAC range shows distinctly different dependence for slow and fast solar wind, suggesting the importance of the composition and properties of SW in controlling the FAC strengths.

Solar Angular Momentum Loss over the Past Several Millennia

Abstract The Sun and Sun-like stars lose angular momentum to their magnetized stellar winds. This braking torque is coupled to the stellar magnetic field, such that changes in the strength and/or geometry of the field modifies the efficiency of this process. Since the space age, we have been able to directly measure solar wind properties using in situ spacecraft. Furthermore, indirect proxies such as sunspot number, geomagnetic indices, and cosmogenic radionuclides, constrain the variation of solar wind properties on centennial and millennial timescales. We use near-Earth measurements of the solar wind plasma and magnetic field to calculate the torque on the Sun throughout the space age. Then, reconstructions of the solar open magnetic flux are used to estimate the time-varying braking torque during the last nine millennia. We assume a relationship for the solar mass-loss rate based on observations during the space age which, due to the weak dependence of the torque on mass-loss rate, does not strongly affect our predicted torque. The average torque during the last nine millennia is found to be 2.2 × 1030 erg, which is comparable to the average value from the last two decades. Our data set includes grand minima (such as the Maunder Minimum), and maxima in solar activity, where the torque varies from ∼1 to 5 × 1030 erg (averaged on decadal timescales), respectively. We find no evidence for any secular variation of the torque on timescales of less than 9000 yr.

Effect of Rapid Changes of Solar Wind Conditions on the Pickup Ion Velocity Distribution

AbstractWe performed the first systematic analysis of pickup ion (PUI) cutoff speed variations, across compression regions and due to fast fluctuations in solar wind (SW) speed and magnetic field strength. This study is motivated by the need to remove or correct for systematic effects on the determination of the interstellar flow longitude based on the longitudinal variation of the PUI cutoff. Using 2007–2014 STEREO A PLASTIC observations, we identified SW compression regions and accumulated the contained PUI velocity distributions in a superposed epoch analysis. The shift of the cutoff in velocity, interpreted as PUI energization, varies systematically across the compression region and increases approximately linearly with the speed gradient of the compression. Additionally, the shift remains positive into the negative speed gradient at the beginning of the rarefaction region. A similar response is found when PUI distributions are sorted according to the strength of fast fluctuations in SW speed, density, and magnetic field strength. These parameters remain high in the first part of the rarefaction region, suggesting a possible PUI energization through compressive turbulence. Based on these results, we removed the strongest compression regions from the interstellar flow analysis, finding no significant change in direction or uncertainty. Thus, we have revealed the influence of adiabatic compression and compressive turbulence, increasing the PUI cutoff energy, and we have demonstrated that the determination of the interstellar inflow direction via analysis of PUI distributions is robust for a multiyear data set, even in the presence of SW interaction regions.

On the response of Martian Ionosphere to the Passage of a Corotating Interaction Region: MAVEN Observations

AbstractThe response of Martian ionosphere to the passage of corotating interaction region (CIR) of June 2015 is studied using observations from several instruments aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. An intense CIR arrived at Mars on 22 June 2015, during which the upstream solar wind and interplanetary magnetic field conditions were monitored by the Solar Wind Ion Analyzer (SWIA), Solar Wind Electron Analyzer (SWEA), Solar Energetic Particle (SEP), and Magnetometer (MAG) instruments aboard MAVEN. The CIR event was characterized by enhancements in solar wind density, velocity, and dynamic pressure, and increased and fluctuating interplanetary magnetic field and was associated with enhanced fluxes of Solar Energetic Particles. The Langmuir Probe and Waves (LPW) instrument onboard MAVEN provided the ionospheric observations such as electron density and electron temperature during this period. The dayside ionosphere is significantly compressed only near the peak of solar wind dynamic pressure enhancement (∼14 nPa). In contrast, on the nightside, the electron density remains depleted for a longer period of time. The electron temperatures are also enhanced during the period of electron depletion on the nightside. The Suprathermal and Thermal Ion Composition (STATIC) measurements show enhanced fluxes of suprathermal heavy ions in the Martian exosphere during CIR period, and evidences for enhanced tailward flow of these pickup ions. The analysis suggests that the nightside ionosphere is primarily controlled by the precipitating Solar Energetic Particles and pickup ions transported across the Martian terminator and depletes significantly when the heavy ion flux in the exosphere enhances.

Building a Weakly Outgassing Comet from a Generalized Ohm's Law.

When a weakly outgassing comet is sufficiently close to the Sun, the formation of an ionized coma results in solar wind mass loading and magnetic field draping around its nucleus. Using a 3D fully kinetic approach, we distill the components of a generalized Ohm's law and the effective electron equation of state directly from the self-consistently simulated electron dynamics and identify the driving physics in the various regions of the cometary plasma environment. Using the example of space plasmas, in particular multispecies cometary plasmas, we show how the description for the complex kinetic electron dynamics can be simplified through a simple effective closure, and identify where an isotropic single-electron fluid Ohm's law approximation can be used, and where it fails.

The low-frequency break observed in the slow solar wind magnetic spectra

Fluctuations of solar wind magnetic field and plasma parameters exhibit a typical turbulence power spectrum with a spectral index ranging between ∼5/3 and ∼3/2. In particular, at 1 AU, the magnetic field spectrum, observed within fast corotating streams, also shows a clear steepening for frequencies higher than the typical proton scales, of the order of ∼3 × 10−1 Hz, and a flattening towards 1/f at frequencies lower than ∼10−3 Hz. However, the current literature reports observations of the low-frequency break only for fast streams. Slow streams, as observed to date, have not shown a clear break, and this has commonly been attributed to slow wind intervals not being long enough. Actually, because of the longer transit time from the Sun, slow wind turbulence would be older and the frequency break would be shifted to lower frequencies with respect to fast wind. Based on this hypothesis, we performed a careful search for long-lasting slow wind intervals throughout 12 years of Wind satellite measurements. Our search, based on stringent requirements not only on wind speed but also on the level of magnetic compressibility and Alfvénicity of the turbulent fluctuations, yielded 48 slow wind streams lasting longer than 7 days. This result allowed us to extend our study to frequencies sufficiently low and, for the first time in the literature, we are able to show that the 1/f magnetic spectral scaling is also present in the slow solar wind, provided the interval is long enough. However, this is not the case for the slow wind velocity spectrum, which keeps the typical Kolmogorov scaling throughout the analysed frequency range. After ruling out the possible role of compressibility and Alfvénicity for the 1/f scaling, a possible explanation in terms of magnetic amplitude saturation, as recently proposed in the literature, is suggested.

Relationships between the solar wind magnetic field and ground-level longwave irradiance at high northern latitudes

Longwave irradiances measured from two sites at different geomagnetic latitudes show different responses to changes in the east-west component of the interplanetary magnetic field (IMF By). At Barrow, Alaska, geomagnetic latitude 69-70oN, neither downward longwave irradiance from the atmosphere nor upward longwave irradiance from the ground show a significant correlation with By. However, at Alert, Canada, geomagnetic latitude near 87oN, a negative correlation that is marginally significant at the 95% level of confidence exists between By and downward longwave irradiance measured 3 days later. On average, a +3.5 nT increase in By is followed by a daily-mean downward longwave irradiance that is smaller by −0.60 ± 0.60% than would exist for a constant By. Similarly, daily-mean upward irradiance at a lag of 4 days is −0.51 ± 0.30% smaller than would exist otherwise, where error bars denote the 95% confidence range. The difference in upward irradiance corresponds to a surface cooling at Alert of approximately 0.33 ± 0.19 K. These results are qualitatively consistent with a previously proposed mechanism in which the interplanetary magnetic field perturbs the ionosphere-to-ground potential difference and the downward atmospheric current density over limited regions near the geomagnetic poles, altering local cloud properties. We find that the atmospheric longwave emission is altered on a time scale of 3 days, with a change in surface temperature appearing one day later, attributable to the thermal inertia of the surface. When one moves from the geomagnetic latitude of Alert (3° from the north geomagnetic pole) to the latitude of Barrow (∼20° from that pole), any connection between By and longwave irradiance becomes too small to isolate from the natural background variability.