Microdialysis Probe

Xxixth Iau General Assembly, August 2015

Piero Benvenuti, ed.

From Solar Orbiter And Parker Solar Probe

Robert F. Wimmer-Schweingruber1 and Javier Rodriguez-Pacheco2 and George C. Ho3 and Christina M. Cohen4 and Glenn M. Mason5

And

the Solar Orbiter EPD and Parker Solar Probe ISIS teams

24118 Kiel, Germany

2Universidad de Alcal´a, Space Research Group, 28805 Alcal´a de Henares, Spain 3Southwest Research Institute, San Antonio, TX, USA 4California Institute of Technology, Pasadena, CA, USA Abstract. The Sun drives a supersonic wind which inflates a giant plasma bubble in our very local interstellar neighborhood, the heliosphere. It is bathed in an extremely variable background of energetic ions and electrons which originate from a number of sources. Solar energetic particles (SEPs) are accelerated in the vicinity of the Sun, whereas shocks driven by solar disturbances are observed to accelerate energetic storm particles (ESPs). Moreover, a dilute population with a distinct composition forms the anomalous cosmic rays (ACRs) which are of a mixed interstellar- heliospheric origin. Particles are also accelerated at planetary bow shocks. We will present recent observations of energetic particles by Solar Orbiter and Parker Solar Probe, as well as other spacecraft that allow us to study the acceleration and transport of energetic particles at multiple locations in the inner heliosphere.

Keywords. sun, heliosphere, energetic particles, Solar Orbiter, Parker Solar Probe

1. Introduction

The Integrated Science Investigation of the Sun (IS⊙IS,

Mccomas Et Al. 2016) On

Parker Solar Probe (PSP, Fox et al. 2016) and the Energetic Particle Detector (EPD, Rodr´ıguez-Pacheco et al. 2020) on Solar Orbiter (M¨uller et al. 2020) basically have the same science goals which are worded slightly differently and are summarized here for convenience. McComas et al. (2016) states IS⊙IS’ top level science goal as to determine the physical mechanisms that produce, accelerate, and transport energetic particles in the inner heliosphere. As usual, this is broken down into three lower-level science questions: a) What is the origin or seed population of solar energetic particles (SEPs)? b) How are these SEPs and other particle populations accelerated? c) What mechanisms are responsible for transporting the different particle populations into the heliosphere? On the other hand, EPD ’s top-level science question is (see Rodr´ıguez-Pacheco et al. 2020) to determine how solar eruptions produce energetic particle radiation that fills the heliosphere. This is also broken down into three more detailed science questions. a) How and where are energetic

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Wimmer-Schweingruber et al. particles injected at the sources and, in particular, what are the seed populations for energetic particles? b) How and where are energetic particles accelerated at the Sun and in the interplanetary medium? c) How are energetic particles released from their sources

And Distributed In Space And Time?

Obviously there are three common science questions which can be broken down into three processes which are needed for energetic particles to be measured in the heliosphere: injection (sec. 2), acceleration (sec. 3), and transport (sec. 4). We have chosen to structure this paper according to these three processes and to provide examples for these processes that use data from Solar Orbiter, PSP, and other spacecraft that were recently reported.

Because we have been tasked by the organizers of this workshop with providing an overview of results from PSP and Solar Orbiter, this paper does not report brand new results. Rather, it attempts to exemplify how the new instruments on these missions as well as the various constellations offered by them and other inner-heliosphere missions can elucidate these three important contributions to accelerating energetic particles that fill the heliosphere. As usual, we will provide a summary and discussion at the end of the paper.

2. Injection

In order to do their mischief in the heliosphere, energetic particles need to be injected into the acceleration process. If they are accelerated at a shock, their speed needs to exceed the thermal speed of the local particle population so that they can freely cross the acceleration region (e.g., a shock) multiple times. Just how they attain this elevated kinetic energy has been a highly debated topic in the past decades. Today we know that the solar wind does not serve as the sole or main reservoir for suprathermal particles but that there appears to be a population of such particles available for further acceleration.

An important clue came from measurements of the composition of suprathermal particles. They are enriched in the rare 3He isotope throughout the solar activity cycle (but do show a dependence on it, Wiedendeck and Mason (2013)). Desai and Giacalone (2016) provide an excellent overview of the measurements and their consequences for interpretation.

As is well known, the Sun is an active star with an activity cycle driven by the solar dynamo (Cliver et al. 2022). This is illustrated in the upper panel of Fig. 1 which shows sunspot number (Source: WDC-SILSO, Royal Observatory of Belgium, Brussels) which is a good indicator for solar activity as a function of time. The bottom panel shows oxygen differential flux spectra accumulated over the four Bartels rotations indicated by vertical lines in the upper panel and in the title of the Figure. The line styles in the lower panel correspond to those in the upper panel. Particle data were taken from three instruments on NASA’s Advanced Composition Explorer (ACE, Stone et al. (1998c)), the Ultra-Low-Energy Isotope Spectrometer (ULEIS, Mason et al. (1998)) the Solar Isotope Spectrometer (SIS, Stone et al. (1998b)), and the Cosmic-Ray Isotope Spectrometer (CRIS, Stone et al. (1998a)).

Fig. 1 also shows the highest and lowest particle fluences observed during the course of the ACE mission and illustrates the enormous variability of particle fluxes which is bounded by the shaded area. It covers a dynamic range of about seven orders of magnitude even when averaged over entire Bartels rotations. Some further explanations are needed to interpret the figure. Fluxes at the highest energies covered by ULEIS during quiet times are affected by instrumental noise which is due to a malfunction of the second time-of-flight (ToF) measurement. The measured values (from the ACE Science Center) are plotted as a line plot with a “+” symbol. Because we know that the lower limits are not correct over the entire energy range, we have also plotted a logarithmic interpolation

Cris

O spectra for Bartels rotations 2000.869, 2004.713, 2009.074, and 2011.663 Figure 1. Upper panel: Sunspot numbers vs. time (from WDC-SILSO, Royal Observatory of Belgium, Brussels). Bottom panel: Oxygen differential flux spectra accumulated over the time periods indicated by vertical lines in the upper panel (decimal years 2000.869, 2004.787, 2009.074, and 2011.663). Line styles in the lower panel indicate the Bartels rotation of the observation as indicated in the upper panel. Colors indicate different instruments on the ACE spacecraft as discussed in the text. The shaded region shows the envelope of the variability of the Bartels-ro- tation-averaged O particle fluences from 1998 – 2024. See text for an explanation of the “x” and “+” symbols.

between the lowest-energy SIS data point and the ULEIS data point with the highest energy which is not affected by this background (around 1 MeV/nuc). Moreover, the CRIS instrument on ACE was not designed to measure high fluxes during solar particle events but to provide accurate measurements of the quiet-time galactic cosmic rays. It saturates during high-flux time periods† which is seen by the measured data points which are also shown as a line plot with an “x” symbol marking. To establish a likely envelope, shown as the solid line bounding the shaded region, we have used a fluence of 0.8 times the highest-energy SIS data point and 0.8 times the square root of that value multiplied by the second-lowest CRIS fluence value. This is also the reason for the “missing” CRIS data in Fig. 1. Figure 1 and its discussion above are updated versions of a similar figure shown in Wimmer-Schweingruber et al. (2023b).

Here we illustrate the injection stage with new measurments performed with the Suprathermal Ion Spectrograph (SIS Rodr´ıguez-Pacheco et al. 2020) on Solar Orbiter. SIS

Communication,

2022.

At

https://izw1.caltech.edu/ACE/ASC/level2/cris_l2desc.html

“Time

periods during which solar activity is high are not covered by CRIS, since the instrument was

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Wimmer-Schweingruber et al. measures total energy of and performs multiple time-of-flight measurements on suprather- mal particles. This results in a very low background which allows measurements of the heliospheric quiet-time background of suprathermal particles (Mason et al. 2021, 2023).

We will not consider those measurements here but rather investigate the origin of parti- cles injected into and accelerated during two impulsive solar particle events observed by Solar Orbiter on the 9th of November 2022 and on the 8th of April 2023, as reported by Mason et al. (2023). The first of these events was very unusual in that it contained several hundreds of ultraheavy ions (with masses > 78 amu). The second event occurred during very low ambient activity which allowed those authors to observe particle spectra with only minimal pre-event background from the suprathermal seed population. These ob- servations showed very low proton and 4He intensities together with intensities of other ions that are typical of 3He-rich events. Together these observations indicate that the enrichments in Fe/O, 3He/4He, etc. which have often been attributed to the injection process are more likely due to the acceleration process itself. In the model of Roth and Temerin (1997) and Temerin and Roth (1992) ions are accelerated by electromagnetic hydrogen cyclotron waves in a single-stage process. Ions are heated over a broad range of gyrofrequencies, but in the model presented by Mason et al. (2023) the spectrum of waves is strongly damped at the gyrofrequencies of the abundant H and 4He. Thus the “unusual” abundance features such as enhanced Fe/O is not due to an enrichment of Fe but rather to a depletion of O because it is fully stripped and thus in resonance with the waves damped by 4He. This process would also explain the enrichments of the neutron- rich isotopes of Ne, Mg, Si, etc. which have been reported in the past (Dwyer et al. 2001; Leske et al. 2001, 2007; Mason et al. 1994).

3. Acceleration

Acceleration of particles requires work to be done on them which in turn requires an electric field. Strong electric fields are difficult to maintain in a highly conductive plasma such as the corona or solar wind. Maxwell’s equations allow a way out of this dilemma by inducing electric fields with a rapidly changing magnetic field. Rapid temporal vari- ations can be achieved in solar flares as a consequence of reconnection. Rapid spatial variations such as shocks or other magnetic discontinuities translate to rapid tempo- ral changes as they are convected into a plasma. Finally waves traveling through the plasma can also heat particles which are in resonance. Yang et al. (2023) and Trotta et al. (2023) have investigated the acceleration of suprathermal ions using data from the SupraThermal Electron Proton (STEP) sensor which is part of EPD on Solar Orbiter and offers unusually high energy and time resolution (Rodr´ıguez-Pacheco et al. 2020; Wimmer-Schweingruber et al. 2021; Yang et al. 2023).

Yang et al. (2023) reconstructed the pitch-angle distributions (PADs) of suprathermal ions in the solar wind frame of reference and studied its temporal evolution, as well as the flux profile and velocity distribution function of these particles in the vicinity of a shock observed on the 3rd of November 2021. They found that suprathermal proton fluxes peaked ∼12 to ∼24s before the shock in the upstream region, and that close to the shock they exhibited anisotropic PADs that pointed away from the shock and tended to isotropize farther upstream. Downstream of the shock, the PADs showed anisotropies towards a pitch angle of 90◦. Comparison of their measurements with theoretical models suggests that the acceleration of suprathermal protons at interplanetary shocks is highly dynamic on a timescale of ∼10s, i.e., few proton gyroperiods. Furthermore, they con- clude that shock-drift acceleration likely plays an important role in accelerating these suprathermal protons. They suggest that the strong variability of the suprathermal par-

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ticle fluxes could be a result of time-varying acceleration efficiency at the shock or that it is a consequence of shock rippling (Johlander et al. 2016, 2018; Yang et al. 2018), i.e., a rapidly varying magnetic connection to different parts of the shock that would have different local θBn and therefore different acceleration efficiencies.

Trotta et al. (2023) combined results from hybrid kinetic simulations with STEP mea- surements of suprathermal ions in the vicinity of the 30 October 2021 shock to show that the inherent variability of the injection process in both time and space must be considered to solve the problem of how suprathermal particle injection occurs in astro- physical systems. In other words, their results strongly support the second explanation given by Yang et al. (2023). Similar to that work, they found highly variable particle enhancements but in this case they showed clear velocity dispersion in the vicinity of the shock on time scales of 10 to 20 s. In their view, the particles are accelerated at different locations of the shock which are magnetically connected to the spacecraft at different times. These authors also used 2.5-dimensional kinetic simulations to model details of the shock and particle heating and acceleration. They found that suprathermal particles are not distributed uniformly in the vicinity of the shock. Their flux varies strongly along the shock front which indicates irregular injection into the acceleration process.

Together the results of Yang et al. (2023) and Trotta et al. (2023) underline the im- portance of detailed, high-resolution studies in the vicinity of interplanetary shocks to understand both the injection and acceleration of particles. They also drive home the point that injection (Sec. 2) and acceleration (Sec. 3) are not isolated processes but need to be considered together. We could just as well have presented these results in Sec. 2!

4. Transport

Particles can escape the injection/acceleration region after which we speak of “trans- port”. A simple thought experiment starting with a point-like source may serve to illus- trate this process. If we assume a point-like source then particles can propagate along the interplanetary magnetic field (IMF), they can experience drifts and diffusion perpen- dicular to the IMF, and further energy and momentum changes due to changes in the IMF at all temporal and spatial scales. Finally, flux tubes filled with energetic particles to different levels can be convected by an observer which results in interesting temporal behavior (see, e.g., Wimmer-Schweingruber et al. 2023a). To illustrate these effects we will discuss four publications by Trotta et al. (2024), Telloni et al. (2021), Kollhoff et al.

(2021), and Mason et al. (2021). How can we decide whether an observed feature in a particle event is due to injec- tion/acceleration or due to transport? To answer that we need to know how the source, e.g., a shock evolves in the inner heliosphere where it accelerates particles. Trotta et al.

(2024) observed a CME-driven interplanetary shock at 0.07 with Parker Solar Probe and at 0.7 with Solar Orbiter on the 5th of September 2022. This allowed them to study the shock properties at these two different heliocentric distances. Coincidentally, the 0.07 au or 15 R⊙are also the closest to the Sun that a shock has been observed to date. PSP (So- lar Orbiter) was located at Carrington longitude 232◦(252◦) and latitude -1.8◦(-3.6◦).

While this is not a perfect alignment the measured shock properties were nevertheless very similar at the two locations: θBn was 53◦(51◦), shock magnetic compression ratio, rB, was 2.3 (1.9), and the gas compression ratios was 1.6 (2.0). Shock speed decreased from 1520 km/s at PSP to 942 km/s at Solar Orbiter, but the fast magnetosonic and Alfv´enic Mach numbers were very similar (Mfms = 3.8

(3.2) And Ma = 3.9

(3.8)). Despite these similarities, the shock at PSP was seen as a sharp transition without any signs of an upstream foreshock, but much more complex at Solar Orbiter. There a

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