Magnetic Field Intensity Modification to Force Free Model of Magnetic Clouds: Website of Wind Examples From Launch to July of 2015

We describe a new NASA website that shows normalized magnetic field (B) magnitude profiles within Wind magnetic clouds (MCs) (i.e., observations versus basic model versus modified model) for 209 MCs observed from launch in late 1994 to July of 2015, where model modification is based on the studies of Lepping et al. (Solar Phys, 2017, 292:27) and Lepping et al. (Solar Phys, 2018, 293:162); the basic force free magnetic cloud parameter fitting model employing Bessel functions (Lepping et al., J. Geophys. Res., 1990, 95:11957) is called the LJB model here. The fundamental principles should be applicable to the B-data from any spacecraft at 1 AU. Earlier (in the LJB study), we justified why the field magnitude can be thought of as decoupled from the field direction within an MC, and further, we justified this idea in terms of actual observations seen over a few decades with examples of MCs from Wind data. The model modification is achieved by adding a correction (“Quad”) value to the LJB model (Bessel function) value in the following manner: B (est)/B0 ≈ [LJB Model + Quad (CA,u)], where B0 is the LJB-estimated field magnitude value on the MC’s axis, CA is the relative closest approach (See Supplementary Appendix A), and u is the distance that the spacecraft travels through the MC from its entrance point. In an average sense, the Quad technique is shown to be successful for 82% of the past modeled MCs, when Quality (Q0) is good or excellent (see Supplementary Appendix A). The Quad technique is successful for 78% of MCs when all cases are considered. So Q0 of the MC LJB-fit is not a big factor when the success of the Quad scheme is considered. In addition, it is found that the Quad technique does not work better for MC events with higher solar wind speed. Yearly occurrence frequency of all MC events (NYearly) and those MC events with ΔσN/σN2 ≥ 0.5 (NΔσN/σN2≥0.5) are well correlated, but there is no solar cycle dependence for normalizing NΔσN/σN2≥0.5 with NYearly.

Response to “Limitations in the Hilbert Transform Approach to Locating Solar Cycle Terminators” by R. Booth

Booth (Solar Phys.296, 108, 2021; hereafter B21) is essentially a critique of the Hilbert transform techniques used in our paper (Leamon et al., Solar Phys.295, 36, 2020; hereafter L20) to predict the termination of solar cycles. Here we respond to his arguments; our methodology and parameter choices do extract a mathematically robust signature of terminators from the historical sunspot record. We agree that the attempt in L20 to extrapolate beyond the sunspot record gives a failed prediction for the next terminator of May 2020, and we identify both a possible cause and remedy here. However, we disagree with the B21 assessment that the likely termination of Solar Cycle 24 is two years after the date predicted in L20, and we show why.

A journey of exploration to the polar regions of a star: probing the solar poles and the heliosphere from high helio-latitude

A mission to view the solar poles from high helio-latitudes (above 60°) will build on the experience of Solar Orbiter as well as a long heritage of successful solar missions and instrumentation (e.g. SOHO Domingo et al. (Solar Phys. 162(1-2), 1–37 1995), STEREO Howard et al. (Space Sci. Rev. 136(1-4), 67–115 2008), Hinode Kosugi et al. (Solar Phys. 243(1), 3–17 2007), Pesnell et al. Solar Phys. 275(1–2), 3–15 2012), but will focus for the first time on the solar poles, enabling scientific investigations that cannot be done by any other mission. One of the major mysteries of the Sun is the solar cycle. The activity cycle of the Sun drives the structure and behaviour of the heliosphere and of course, the driver of space weather. In addition, solar activity and variability provides fluctuating input into the Earth climate models, and these same physical processes are applicable to stellar systems hosting exoplanets. One of the main obstructions to understanding the solar cycle, and hence all solar activity, is our current lack of understanding of the polar regions. In this White Paper, submitted to the European Space Agency in response to the Voyage 2050 call, we describe a mission concept that aims to address this fundamental issue. In parallel, we recognise that viewing the Sun from above the polar regions enables further scientific advantages, beyond those related to the solar cycle, such as unique and powerful studies of coronal mass ejection processes, from a global perspective, and studies of coronal structure and activity in polar regions. Not only will these provide important scientific advances for fundamental stellar physics research, they will feed into our understanding of impacts on the Earth and other planets’ space environment.

High-speed shear-driven dynamos. Part 1. Asymptotic analysis

Rational large Reynolds number matched asymptotic expansions of three-dimensional nonlinear magneto-hydrodynamic (MHD) states are the concern of this contribution. The nonlinear MHD states, assumed to be predominantly driven by a unidirectional shear, can be sustained without any linear instability of the base flow and hence are responsible for subcritical transition to turbulence. Two classes of nonlinear MHD states are found. The first class of nonlinear states emerged out of a nice combination of the purely hydrodynamic vortex/wave interaction theory by Hall & Smith (J. Fluid Mech., vol. 227, 1991, pp. 641–666) and the resonant absorption theories on Alfvén waves, developed in the solar physics community (e.g. Sakurai et al. Solar Phys., vol. 133, 1991, pp. 227–245; Goossens et al. Solar Phys., vol. 157, 1995, pp. 75–102). Similar to the hydrodynamic theory, the mechanism of the MHD states can be explained by the successive interaction of the roll, streak and wave fields, which are now defined both for the hydrodynamic and magnetic fields. The derivation of this ‘vortex/Alfvén wave interaction’ state is rather straightforward as the scalings for both of the hydrodynamic and magnetic fields are identical. It turns out that the leading-order magnetic field of the asymptotic states appears only when a small external magnetic field is present. However, it does not mean that purely shear-driven dynamos are not possible. In fact, the second class of ‘self-sustained shear-driven dynamo theory’ shows a magnetic generation that is slightly smaller in size in the absence of any external field. Despite its small size, the magnetic field causes the novel feedback mechanism in the velocity field through resonant absorption, wherein the magnetic wave becomes more strongly amplified than the hydrodynamic counterpart.

Nonlinear theory of non-axisymmetric resonant slow waves in straight magnetic flux tubes

Nonlinear resonant slow magnetohydrodynamic (MHD) waves are studied in weakly dissipative isotropic plasmas for a cylindrical equilibrium model. The equilibrium magnetic field lines are unidirectional and parallel with the z axis. The nonlinear governing equations for resonant slow magnetoacoustic (SMA) waves are derived. Using the method of matched asymptotic expansions inside and outside the narrow dissipative layer, we generalize the connection formulae for the Eulerian perturbation of the total pressure and for the normal component of the velocity. These nonlinear connection formulae in dissipative cylindrical MHD are an important extention of the connection formulae obtained in linear ideal MHD [Sakurai et al., Solar Phys. 133, 227 (1991)], linear dissipative MHD [Goossens et al., Solar Phys. 175, 75 (1995); Erdélyi, Solar Phys. 171, 49 (1997)] and in nonlinear dissipative MHD derived in slab geometry [Ruderman et al., Phys. Plasmas4, 75 (1997)]. These generalized connection formulae enable us to connect the solutions at both sides of the dissipative layer without solving the MHD equations in the dissipative layer. We also show that the nonlinear interaction of harmonics in the dissipative layer is responsible for generating a parallel mean flow outside the dissipative layer.

Observable Signals of Coronal Heating Processes

The solar corona is thought to be sustained by waves, currents, turbulence or by velocity filtration. For efficient wave heating of the corona, only the Alfven waves seem to survive the effects of steepening and shock dissipation in the chromosphere (Zirker, 1993, Solar Phys. 148,43) and these can be dissipated in the corona by mode conversion or phase mixing (Priest, 1991 in XIV Consultation on Solar Physics, Karpacz). Enhanced line width of 530.3 nm coronal line seen within closed structures (Singh et al., 1982, J. Astrophys. Astron. 3,248), association of enhanced line width of HeI 1083 nm line with enhanced equivalent width (Venkatakrishnan et al., 1992, Solar Phys. 138,107), and gradients seen in the MgX 60.9 and 62.5 nm coronal line width (Hassler, et al., 1990, Astrophys. J. 348, L77), are possibly some examples of the observed signals of wave heating. Current sheets, produced in a variety of ways (Priest and Forbes, 1989, Solar Phys. 43,177; Parker, 1979, Cosmical Magnetic Fields, Ox. Univ. Press), can dissipate and provide heat. The properties of current sheets can be inferred from fill factors, emission measures (Cargill, 1994, in J.L. Burch and J.H. Waite, Jr. (eds.) Solar System Plasma Physics: Resolution of Processes in Space and Time, AGU Monograph), hard xrays (Lin et al., 1984, Astrophys. J. 283,421), and radio bursts (Benz, 1986, Solar Phys. 104,99). The association of large scale currents with enhanced transition region (deLoach et al., 1984, Solar Phys. 91,235.) and regions of enhanced magnetic shear with brighter corona (Moore et al., 1994, Proc. Kofu Symp) are of some possible interest in this context. Self consistent calculations of the turbulent cascade of energy from the scales of photospheric motions down into dissipative scales (Heyvaerts and Priest, 1992, Astrophys. J. 390,297) predict the width of coronal lines as a function of the properties of the forcing flows. Velocity filtration caused by free streaming effects off a non maxwellian boundary distribution of particles may well result in a plasma having coronal properties (Scudder, 1992a, Astrophys. J. 398,299; 1992b, Astrophys.J.11 398,319). The observable signals are the variation of line shapes with altitude.

A Relationship Between the Brightness Temperatures for Type III Bursts

(Solar Phys.). The widely accepted emission mechanisms for type III bursts involve at least two stages. The first stage is the generation of Langmuir waves by the inferred stream of electrons. Emission at the fundamental frequency arises when these waves are scattered by thermal ions. Emission at the second harmonic arises when two Langmuir waves coalesce; however, the coalescence is possible only after an intermediate stage in which the distribution of Langmuir waves evolves towards isotropy due to scattering by thermal ions.

High Resolution Observations of Generalized Fast Drift Bursts

(Solar Phys.). During 1969 and 1970 groups of generalized fast drift bursts were observed on 21 days with a high resolution radio spectrograph at Oslo Solar Observatory. Totally 48 groups were detected in the frequency band 310–340 MHz. In the great majority of the cases the groups were accompanied by metre wave type III bursts at lower frequencies.

The Coronal Disturbance of 12 August 1972

(Solar Phys.). The association of flare sprays, distortions of the overlying coronal structures and moving type IV radio bursts is a reasonable one and is well accepted despite the paucity of observational evidence. On 12 August 1972 there occurred a flare spray observed optically by both flare patrol (National Oceanographic and Atmospheric Administration) and coronagraph (High Altitude Observatory) instruments. A subsequent moving type IV radio burst was recorded on two swept frequency interferometers (Universities of Colorado and Maryland). In addition distinct changes in the K-coronal brightness at 1.6 R⊙ were measured (High Altitude Observatory K coronameter). These observations combine to form one of the most complete sequences of measurements yet recorded covering the range from the chromosphere to about 6 R⊙. The separate measurements are discussed and we show that they can be combined to form a relatively simple physical picture of the whole event.

Type II Burst-Sources as Low-VA Regions in the Corona ‘Illuminated’ by Flare-Induced MHD Shocks

(Solar Phys.). The author has proposed a hypothesis that the type II burst-sources may be due to the local enhancement of the flare-produced MHD fast-mode shock wave on encountering the pre-existing low Alfvén velocity regions in the corona (Uchida, 1973). The purpose of the present paper is to show that this may actually be the case by comparing the behavior of the computor-simulated propagation and strengthening of MHD fast-mode (weak) shock with the observed characteristics of type II burst-sources.