The idea of an “Outer solar system probe: to be aimed away from the Sun in the plane of the ecliptic” dates from a report of the “Simpson Committee” of the Space Science Board of the National Academy of Sciences in March of 1960. What is now known as “Interstellar Probe” has matured as a concept for making new discoveries that can be made in no other way, by going places yet to be explored. “Near-future” propulsion capabilities have always been taken as the backdrop for defining the mission requirements, but the real issue is to unite compelling science with engineering and technical reality. With that perspective in mind, the Johns Hopkins University Applied Physics Laboratory has been tasked by the NASA Heliophysics Division to (re-)study the mission and provide a Technical Report to be delivered late 2021 for input to next Solar and Space Physics Decadal Survey. This “pragmatic Interstellar Probe” of the study is a mission through the outer heliosphere and to the nearby “Very Local” interstellar medium (VLISM), uses today’s technology to take the first explicit step on the path of interstellar exploration, and can pave the way, scientifically, technically, and programmatically for more ambitious future journeys (and more ambitious science goals). To enforce these goals the broadly-based engineering requirements include (1) be ready technically to launch no later than 1 January 2030; (2) be able to transmit useful scientific data from 1000 au; (3) require no more than 600 W (electric) at the beginning of the mission and no more than half of that at mission’s end; and, (4) be capable by design of functioning for no less than 50 years. We provide a top-level summary of the current study status. “It isn’t about where we are going.  It’s about the journey out there.”

During the course of its evolution, our Sun and its protective magnetic bubble have plowed through dramatically different interstellar environments throughout the galaxy. The vast range of conditions of interstellar plasma, gas, dust and high-energy cosmic rays on this solar journey have helped shape the solar system that we live in. Today, our protective bubble, or Heliosphere, is likely about to enter a completely new regime of interstellar space that will, yet again, change the entire heliospheric interaction and how it shields us from the interstellar environment. An Interstellar Probe is a mission concept to explore the mechanisms upholding the heliospheric boundary and take the first step beyond our home, into the interstellar cloud to understand the evolutionary journey of our Sun, Heliosphere and Solar System. An international team of scientists and a team of engineers at the Johns Hopkins University Applied Physics Laboratory (APL) are funded by NASA to study pragmatic mission concepts that would make a launch in the 2030’s a reality. The ground breaking science enabled by such a mission spans not only the discipline of Solar and Space Physics, but also Planetary Sciences and Astrophysics. Here, we give an overview of the science discoveries that await along the journey, including the physics of the heliospheric boundary and interstellar medium, the potential for exploration of Kuiper Belt Objects, the circumsolar dust disk and the extra-galactic background light. We will discuss the details of the study, the example payloads, subsystems and mission architectures that would allow humanity to explore where no one has gone before. 

In the Solar system，there are many astronomical objects owning the magnetosphere structure generated by the interaction between their intrinsic magnetic field and Solar wind, including Mercury，Earth，Jupiter，Saturn Uranus，Neptune and some of their satellites. Heliosphere is the Sun’s magnetosphere，which is filled with Solar wind and surrounded by the interstellar medium. According to the data obtained by the existing detectors，the interaction between the Solar wind and the interstellar medium，the distribution of energy neutral atoms and the relative spatial density changes of picked-up particles，the formation mechanism of abnormal cosmic rays，and possible changes in the shape of the heliosphere are introduced. The marginal exploration plan of China’s Solar system exploration mission is given. Two detectors in opposite directions are designed. One will fly towards the nose tip of the heliosphere to conduct comprehensive exploration of the marginal Solar system and its adjacent space; and the other will fly in the opposite direction to fill the gap at the tail boundary of the spherical layer. The knowledge of the space environment of the heliosphere can provide reference for the design of the probe.

In the long journeys to the heliospheric boundary and the interstellar space as envisaged in several ambitious mission concepts of several space agencies, it is imperative to include planetary bodies in the outer solar system as observational targets to enhance their scientific values and appeal to the general public. A good example is the New Horizons mission to Pluto and the Kuiper belt. Such "targets-of-opportunity" are the Uranisan and Neptunian systems, Centaurs, and a few large Kuiper belt objects like 20000 Varuna. This also means that a miniaturized imaging system should be an integral part of the scientific payload on an interstellar spacecraft to the heliospheric boundary. The flyby observations of any of the above targets could be used as a building block of a long-term outer planet exploration program.

In terms of the scientific objectives of Chinese Solar System Boundary Exploration Mission which is being under discussion, scientific payloads of Magnetometer, Particle Package, Dust Analyzer, Imaging Camera, Ultraviolet Imaging Spectrometer and Visible and Infrared Imaging Spectrometer are proposed. The performance requirements and engineering requirements such as mass and power are given. Because of the tight schedule, heritage information of these payloads is also given. For this super far away exploration, preliminary solutions to common key technologies such as miniaturization and low power, high reliability and long lifetime, science data processing and compression are given.

Energetic Neutral Atom (ENA) imaging provides an essential way to remotely probe distant plasma regions in space, especially for imaging energetic protons. Under the support of NSFC major scientific instrument development grant, we have been developing a grid-modulated ENA imager with high sensitivity and resolutions in Peking University (PKU). This ENA imager utilizes thin-window, low-noise, pixelated silicon semiconductor detectors to detect hydrogen (oxygen) atoms at energies downs to ~5 keV (~10 keV). It also adopts a novel ENA imaging technique that combines Fourier-transform imaging and coded-mask imaging, to enable high-sensitivity imaging with about an order of magnitude less resource requirement. Using these new technologies, the PKU ENA imager will be able to measure ENAs produced in the ring current during terrestrial magnetic storms/substorms, with high temporal and spatial resolutions. Furthermore, this ENA imager can be adopted for the interstellar and deep space missions.

This study shows that a supersonic moon shadow of a total solar eclipse can steepen the ionospheric total electron content (TEC) wave on August 21, 2017. A data-adaptive method named Hilbert-Huang transform is employed to examine the nonlinear and non-stationary evolution of the waves. The results show that the TEC wave behaves as a traveling ionospheric disturbance before the totality appearance, turns later into steepening, and breaks eventually. A TEC wave with a period of ~40min and wavelength of ~1,000km propagates mainly in an east-southward direction before the totality appearance. The wave amplitude and scales, respectively, increases and reduce by near ~50% as the moon shadow approaches the western coast of the continental United States. The short-period TEC waves (period ~2min) reveal that the wave may break eventually when the wave gets steeper. The steepness of the TEC wave is reconstructed according to the constructive interference.

St. Patrick’s geomagnetic storm is an interesting geomagnetic storm due to intense solar wind dynamics allowing the study of the ionospheric properties in various regions. This geomagnetic storm event taking place on 17^th March 2015, the onset time occurred at 04:47 UT. The minimum value of SYM/H index reaching -234 nT occurred at 22:47 UT. The ionospheric variations during the storm time on a global scale can be explained via the ionospheric properties. We study the total electron content (TEC) data obtained by the ground-based GPS receivers (GNSS data), JASON-2 satellite, and the SAMI3/RCM simulation. In addition, the O/N₂ data obtained by GUVI/TIMED satellite are compared with the TEC results. Before the storm time, the TEC is high at a subsolar location in approximately equatorial to low latitude regions during the quiet phase of the solar wind. The average values of the O/N₂ ratios were found at low to middle latitudes, while the lower O/N₂ ratios were found at high latitude regions. However, near the onset time on 17^th March 2015, the TEC increased in equatorial to middle latitude regions. The TEC observations showed high values in northern and southern latitudes near the equatorial region after the SYM/H index reached the minimum value. We intend to compare the TEC data with new SAMI3/RCM simulations, which can be applied for other regions that we do not have the data. For the O/N₂ ratios, the region with low value expanded from high to middle latitudes starting from approximately 12:00 UT, while the O/N₂ ratios clearly increased in low latitude regions. The connections between TEC and O/N₂ ratios and the implication from the simulation in this event could be an important key to explain the ionospheric dynamics.

Nighttime plasma irregularities and scintillations in the low-latitude ionosphere are probed by ROCSAT-1, DEMETER, and FORMOSAT-3/COSMIC (F3/C) during the solar maximum of 1999-2004, solar quiet of 2006-2010, and 2007-2010, respectively.  The occurrence probability and the instantaneous total amplitude of Hilbert-Huang transform are used to examine the irregularities probed by ROCSAT-1 and DEMETER.  Generally, the global distribution of the occurrence probability is similar to that of the total amplitude, except the probability of DEMETER yielding anomalous enhancement over the South American sector during May-August, and however cannot be detected by F3/C S4 index.  A detailed study reveals that the extremely low ambient density can cause the anomalous region in May-August during the solar quiet period.  To resolve the discrepancy, we setup a noise level of the total amplitude to define the irregularities, and compute a new global occurrence probability of irregularity of DEMETER.  The results well agree with the S4 index of F3/C.  Thus, irregularity associated with ionospheric space weather can be investigated.  To examine the ionospheric morphology response to lithospheric activities, we study the global location preference of irregularities and S4 persisting continuously for longer than 24 h at middle and low latitudes (within ±60◦ N geomagnetic latitudes) during the occurrence of large earthquakes.

We examine the variation of VFM measurements on board Swarm satellite and associated ULF magnetic field perturbation in the Pc3 range during local night time around two months of the great Gorkha earthquake (M=7.8 on 25 April 2015). It is interesting to note that the VFM geomagnetic data and their first difference indicate abrupt changes prior to 10 days of the earthquake. These abrupt changes are associated with a peak-to-peak amplitude ≥0.3nT/s, occurrence during very magnetic quiet night-times and occur within the Dobrovolsky circular area around the epicentre of the earthquake. Corresponding Pc3 anomaly are seen to increase in amplitude of Pc3 on 16/04/2015 with an absence of signature on the day of the earthquake. On 25/04/2015. Anomalous Pc3s lasts for more than a month post-earthquake. This earthquake in the seismically active terrain of Himalaya resulted in many aftershocks. It is a challenge to associate the changes in the Pc3 anomaly with the actual and or the result of the aftershocks. Attempt is made to elucidate and understand the coupling process among the different regions of the Earth.

Recently, ionospheric anomalies related to earthquakes  are considered promising. In Japan, the previous statistical study of GPS-TEC shows a significant positive anomaly 1-5 days before the earthquakes with M≥6, depth D≤40 km, epicenter distance of 1000 km Also we have reported that the statistical ionosonde NmF2 analysis during 1958-2017 shows also a significant anomaly 1-10 days before the earthquakes with M≥6, depth D≤40 km, epicenter distance R≤350 km, which indicates the maximum electron density anomaly in the F2 layer. However, the sensitivity of NmF2 anomalies for earthquakes is not clarified. Therefore, we performed statistical analysis to investigate the magnitude, depth, and epicenter distance dependences for earthquakes in which there is a significant positive anomaly in NmF2. This study used ionosonde data observed at Kokubunji, Japan, operated by the National Institute of Information and Communications Technology. We defined the anomaly as the value in excess of median +1.5 IQR of the NmF2 at the same hour in the previous 15 days and the anomalous day as ten or more hours of the anomalies appear in one day. We performed the Superposed Epoch Analysis (SEA) to investigate statistical significance in the correlation between NmF2 anomalies and earthquakes. To evaluate the statistical significance, we examined the random SEA test and we found that there is a significant NmF2 anomaly 6-10 days before the earthquakes. NmF2 anomalies are sensitive to earthquakes with M≥5.8, depth D≤40 km, epicenter distance R≤350 km. Furthermore, we performed Molchan’s Error Diagram analysis to evaluate the efficiency of NmF2 anomalies for earthquake forecasting. These results indicated that NmF2 anomalies are a precursor of earthquakes and are more precursory for shallower, larger, and closer earthquakes. The result indicated that the forecast is most efficient when M≥6.4, D≤20 km, R≤200 km, Δ=10, L=1. and detected about 46% of targeted earthquakes.

The China Seismo-Electromagnetic Satellite (CSES) was successfully launched on February 2, 2018. It is a sun-synchronous orbit satellite with an inclination angle of 97.4° at the altitude of 507 km and with an observation range between -65° to 65° of geographical latitudes. The scientific payload, i.e., plasma analyzer package (PAP) onboard the CSES is designed to detect in-situ ionospheric ion parameters. The observation data of PAP include hydrogen ion density, helium ion density, oxygen ion density, ion temperature, ion drift velocity, and ion density fluctuation.Unfortunately, the in-flight commissioning test indicated that the sensors of PAP were contaminated approximately one month after launch, leading to the ion density values are greatly reduced compared with that in the early period, and the ion drift velocity data are larger than the typical values in the orbit position.    However, the long-term data validation shows that the PAP still has ability to discern the ionospheric disturbances through its relative variation trend or feature. Firstly, from the perspective of overall distribution, the global distribution of PAP data in daytime during an entire revisited period explicitly reflect that the ion contents distribute along the magnetic equator. Further, we analyze the ionospheric disturbance phenomenon caused by high-power terrestrial VLF transmitters, the Ms7.3 Venezuela earthquake on 21 August 2018, and the geomagnetic storm event on 26 August 2018, which are observed by PAP. In all, it is confirmed that PAP data can be selectively used to scientific research through analyzing the relative variation. Furthermore, China and Italy are presently building the second satellite, i.e., CSES-02. Based on the current experience, the PAP onboard CSES-02 has been taken a lot of protective measures to prevent contaminations, in order to offer even greater detection potential for ionospheric studies.

In this study, we investigate the characteristics of pre-earthquake ionospheric effects related to the M7.7 Lucea earthquake (19.46°N, 78.79°W) in Jamaica on January 28, and the M7.0 Aegean Sea earthquake (37.92°N, 26.79°E) on October 30 in the year of 2020. To investigate the temporal and spatial distributions of ionospheric anomalies, we first studied the grid total electron content (TEC) data provided by Center for Orbit Determination in Europe (CODE). A 15-day running median of TEC values and the associated inter-quartile range (IQR) are utilized as a reference for identifying abnormal signals. The results exhibited significant positive anomalies in the south of epicenter on January 26 and 27, which were one and two days prior to the Jamaica earthquake, lasting for more than 6 hours. Then, further study is conducted by using the electron density (Ne) observed by Langmuir probe (LAP) onboard the China Seismo-Electromagnetic Satellite (CSES). By adopting the moving median method (MMM), remarkable enhancements were detected on the same days. By comparing the disturbed orbits with their corresponding four nearest and four revisiting orbits as well as the predicted values from the International Reference Ionosphere 2016 model (IRI-2016), the results strongly illustrate that there were prominent ionospheric anomalies preceding this earthquake. Concerning the Aegean Sea earthquake in Turkey, although positive anomalies were also detected on October 26, 4 days before this earthquake, the seismic-related ionospheric anomalies became ambiguous due to the disturbed space weather lasting for one week. Therefore, how to distinguish the global disturbances related to a storm/substorm and local anomalies caused by a seismic event will be a further study in our future work.

The ionospheric disturbance called plasma bubbles that occurs in the equatorial ionosphere has an irregular plasma density structure, so it degrades radio wave propagation from various satellites. Although plasma bubbles have been observed for more than 80 years, the generation mechanism has not been clarified and the prediction of plasma bubble occurrence is quite difficult. In this study, we have developed a new simulation model for studying plasma bubbles. Models that simulate the ionosphere can be roughly categorized into two types: the global ionosphere model and the local ionosphere model. There is a trade-off between computational domain and resolution. The High-Resolution Bubble (HIRB) model, which is one of the local ionospheric models, can reproduce plasma bubbles in 1 km resolution, but the simulation domain is limited to a narrow wedge region. The model developed in this study is a multi-scale numerical model which covers the whole longitude with a high resolution domain in the dusk region. The new model has the advantages of both global and local models and compensates both disadvantages. In the multi-scale simulation model, plasma bubbles are generated in the high-resolution domain and penetrate into the topside ionosphere. Although the resolution is not as high as the HIRB model, the generated plasma bubbles contain irregular plasma density structures. Plasma drift velocity simulated in the new model are consistent with observations in the all local time region. It indicates that the global simulation is well performed as well as the plasma bubble generation. In this study, the new simulation model has been developed to simulate the plasma bubble generation and global plasma drift velocity in the whole longitude with a multi-scale grid system.

This study presents collaborative observations of the equatorial plasma bubbles (EPBs) by L-band scintillation, density fluctuation, vertical plasma drift, and airglow emission at various altitudes in the F layer using the COSMIC-2, SWARM, ICON, and GOLD data. The result shows that locating the EPBs by in-situ measurements highly dependent on the orbit inclination, thus the EPB occurrence detected by COSMIC-2, SWARM, and ICON display distinct pattern in both temporal and spatial aspects. These in-situ detections are well collocated with the 135.6nm EIA emission gaps in the GOLD monitoring images at the Atlantic-Brazilian longitude sector. The constitution of the COSMIC-2, SWARM, ICON, and GOLD missions provides us a detailed observation of the three-dimensional structure of EPBs. The preliminary results of the vertical drift velocity measured by the ion velocity meter (IVM) at/within the detected EPBs are also compared with the SAMI-3 model.

The ionospheric delay derived from the dual-frequency GPS observations is sometimes not available under the severe plasma bubble event due to the weaker L2 signal,  making it difficult to observe the entire evolution and impact of plasma bubbles. In this study, the ionospheric delay is estimated by the single difference (SD) method, which combines the single-frequency carrier phase (L1) and pseudo-range (C1) measurements between two ground-based receivers. However, using the single-frequency algorithm, the relatively robust L1 signal is still available to investigate the time evolution and spatial distribution of plasma bubbles. As the methodology involves carrier phase observations, solving the phase ambiguity is essential for retrieving the correct ionospheric delay. The ambiguity is solved by applying the Kalman filter for preliminary estimation followed by the well-known LAMBDA (Least square AMBiguity Decorrelation Adjustment) algorithm to approach the best integer solution. The developed algorithm is applied to a hundred stations over Taiwan, located at the low latitude ionosphere. The ionospheric gradient will be larger here when the plasma irregularities occurred that could degrade the positioning accuracy. We examine the ionospheric gradient during a period of magnetic storm and plasma irregularities in March 2015 to understand the evolution of plasma bubbles and their impact on the application of differential-based positioning (such as DGPS and RTK) that requires reference stations.  The results capture the evolution of plasma bubbles and show good consistency with the all-sky airglow observation while the loss of L2 signal, demonstrating the benefits of this algorithm.  The maximum ionospheric gradient reaches ~200 mm/km, indicating the assumption of the same ionospheric condition even within the short-baseline is no longer valid when the plasma bubbles are present. Furthermore, the SD ionospheric delays can be used to mitigate the ionospheric errors for the differential-based positioning.

The day-to-day variability of global low-latitude electron density distributions is investigated by using Global Ionosphere Specification (GIS) electron density profiles derived from the radio occultation measurements by FORMOSAT-7/COSMIC-2 (F7/C2) satellites during a one-year period from August 2019 to July 2020. These daily electron density profiles reveal significant day-to-day changes over dayside low latitudes that vary with seasons, with standard deviations of about 10-20% in equinoxes, 20-30% in solstices, with 40-50% in winter. At night, the percentage standard deviation increases to about 30-60%, with largest values in solstices. Over latitudes of the equatorial ionization anomaly (EIA), the average deviation ranges around15-30% at 1400 LT when the density is usually at a maximum. The period investigated in this study, under deep solar minimum conditions, has remained mostly geomagnetically quiet except for some occasional minor-to-moderate disturbances, and thus much of this variability is likely to be attributed to vertical coupling with the lower atmosphere. To better understand possible driving forces that contribute to this observed day-to-day variability, tidal analysis of the GIS electron density distribution is carried out. Unlike in earlier studies, the results reveal the presence of strong DW1 response over the EIA crest latitudes. The reconstructed amplitudes by using migrating tide components amount to about 50-75% of the overall electron density variation.  Conversely, the non-migrating DE2, DE3, SPW3, SPW4, SE1, and SE2 components induce significant day-to-day variations in the observed wave-3 or wave-4 longitudinal structures in a fixed local-time frame. The magnitude of the net contribution to the overall longitudinal wave structure depends on the relative amplitudes and phases of these non-migrating tidal components. The results further show that, besides these non-migrating components, contribution from other tidal modes add to the complexity of the daily longitudinal patterns in electron density.

This study presents atmospheric lunar tide simulations at mesosphere-lower-thermosphere heights performed by a coupled ocean-atmosphere primitive equation model. The atmospheric model is a non-linear, time-dependent and high-top spectral model. In the atmospheric model, the ocean and load tides are forced by incorporating the ocean and load tide elevation fields from the Finite Element Solution 2014 (FES2014) global ocean atlas model. In addition, the gravitational components of the M2 and N2 lunar tides are incorporated by specifying the geopotential perturbations arising from the lunar gravitational potential field, the earth tide, and the tidal-induced redistribution of mass of the earth’s crust. To ensure realistic tidal propagation conditions, the modeled mean zonal winds and temperatures are nudged to data from the Navy Global Environmental Model - High Altitude (NAVGEM-HA) meteorological analysis system up to thermospheric heights (~95 km altitude). We present simulation results of longitudinal and latitudinal lunar tide variations, as well as simulations of inter-annual variability based on four years of simulations between 2014 and 2017.

Rocket launches carrying satellites into orbit become more frequently nowadays, and their effects to the ionospheric electron density draw attentions with increasing available observations. Observations showed that various traveling ionospheric disturbances (TIDs) could result from the rocket-triggered shock acoustic waves (SAWs) or gravity waves (GWs). Their characteristics can be identified from the shape, period, and phase velocity of TIDs. The reported TIDs generally have a V-shape signature with propagating velocity over the sound speed for SAWs. On the other hand, the TIDs generated from GWs usually reveal a circular- or arc-shape with the period and wavelength in the range of 8-13 min 200-500 km. In this study, we summarize the characteristics of TIDs triggered by several rocket launches, particularly from SpaceX between 2017 and 2020 in America. The ionospheric total electron content (TEC) retrieved from ground-based Global Navigation Satellite System (GNSS) receivers are used to extract the rocket-triggered TIDs by a band-pass filter with a period of 8 to 15 mins. Our results indicate that the behaviors of TIDs triggered by the rocket are highly dependent on the launch conditions, e.g. vertical or horizontal trajectory. In addition, the characteristic of TIDs and their relationship to the background wind field are also discussed.