The Need for Near-Earth Multi-Spacecraft Heliospheric Measurements and an Explorer Mission to Investigate Interplanetary Structures and Transients in the Near-Earth Heliosphere

Abstract

Based on decades of single-spacecraft measurements near 1 au as well as data from heliospheric and planetary missions, multi-spacecraft simultaneous measurements in the inner heliosphere on separations of 0.05-0.2 au are required to close existing gaps in our knowledge of solar wind structures, transients, and energetic particles, especially coronal mass ejections (CMEs), stream interaction regions (SIRs), high speed solar wind streams (HSS), and energetic storm particle (ESP) events. The Mission to Investigate Interplanetary Structures and Transients (MIIST) is a concept for a small multi-spacecraft mission to explore the near-Earth heliosphere on these critical scales. It is designed to advance two goals: (a) to determine the spatiotemporal variations and the variability of solar wind structures, transients, and energetic particle fluxes in near-Earth interplanetary (IP) space, and (b) to advance our fundamental knowledge necessary to improve space weather forecasting from in situ data. We present the scientific rationale for this proposed mission, the science requirements, payload, implementation, and concept of mission operation that address a key gap in our knowledge of IP structures and transients within the cost, launch, and schedule limitations of the NASA Heliophysics Small Explorers program.

Keywords: Coronal mass ejection; Interplanetary space; Mission concept.

Fig. 1

Overview of the key scales of near-Earth interplanetary structures (black bars), transients (blue bars) and energetic particles (orange bars) and the correlation lengths (crimson bars) of energetic storm particle (ESP) events, CME sheaths and ejecta and the size of solar energetic electron (SEE) events. These summarize several dozens studies over the past two decades, as discussed in the text. The MIIST orbit (grey box) is optimized to ensure that most structures, transients, and energetic particles impacting Earth are measured by four identical spacecraft. The various L1 orbits, Wind prograde orbit of 2000–2002 and STEREO near-Earth passes in 2007 and 2023–2024 are shown in grey colors

Fig. 2

SIR on 2007 May 7–8 when the longitudinal separation between STEREO-A (red lines) and STEREO-B (blue lines) was 7^∘. Adapted from Jian et al. (2009). Forward shocks (F.S.) are marked by dashed lines. A reverse shock (R.S.) at STEREO-A is marked by a dashed line. The HCS is marked by a dotted line. The panels show from top to bottom the suprathermal ion flux around 400 keV, the proton velocity, density, temperature, the magnitude of the magnetic field, the east-west magnetic field angle and the total pressure (sum of the magnetic and thermal pressures)

Fig. 3

Past multi-spacecraft measurements of CMEs up to early 2021 (adapted from Lugaz et al. with updates), typical shape of the cross section of a MC following Démoulin et al. (2016), and the 2-spacecraft separations made possible by MIIST. Many such past multi-spacecraft measurements partially rely on planetary missions which often only include magnetic field (no plasma) measurements. STEREO-A in 2022–2023 enabled 10–20 additional 2-spacecraft measurements within the critical region of 0–0.1 au of radial separation and 0–15^∘ of angular separation, which are currently under analysis

Fig. 4

Correlation of the IMF and solar wind speed between STEREO-B and L1 as a function of the longitudinal separation in 2006–2008. The solar wind speed measured at two spacecraft separated by $<{60}^{\circ }$ is well correlated (Pearson correlation, PCC > 0.5), the same does not hold true for the IMF magnitude beyond 20^∘ or for $B_z$ beyond ∼5^∘. Adapted from Bailey et al. (2020)

Fig. 5

In situ measurements of solar wind plasma and magnetic field parameters by STEREO-A and Wind in April and May 2023. Several CMEs and CIRs have impacted both spacecraft, while STEREO-A was positioned 0.05 au closer to the Sun, moving from $-{10.4}^{\circ }$ to $-{8.5}^{\circ }$ longitude (HEEQ) during the plot interval. For corotating structures, this should correspond to STEREO-A measuring streams ∼12–18 hours earlier than Wind

Fig. 6

MIIST Science Traceability Matrix and Payload Requirements

Fig. 7

MIIST spacecraft with payload and field-of-view after deployment. Each MIIST spacecraft rolls slowly during science mode around the Sun-spacecraft line. The magnetometer is on top of 185-cm two-segment boom with additional commercial magnetometers on the spacecraft bus

Fig. 8

MIIST Sun–Earth DRO with $d_{min}$ = 0.048 AU enables four-spacecraft measurements that meet all science requirements. (a) in IAU Sun pole (Earth’s orbit shown with a dashed black line), (b) in GSE coordinates