Recent measurements from the NASA Parker Solar Probe (PSP) revealed that the solar wind emerging from coronal holes, or ‘streams,’ is organized into ‘microstreams’ with an angular scale (5-10°) in Carrington longitude8 similar to the underlying supergranulation cells associated with horizontal flows in the photosphere9. Supergranulation flows are thought to drag the photospheric magnetic field into the convective downflows where the field intensifies and the magnetic field energy density dominates over the plasma pressure2. However, the footpoints of the previous PSP encounter were inferred to be at high latitudes on the far side of the Sun so that the detailed magnetic structure of the cells and their connectivity to the spacecraft could not be determined, preventing a complete analysis of the source of these wind microstreams.
Microstream velocity structure formed at the base of the corona
On solar Encounter 10 (E10) the PSP came within 12.3 solar radii (RS) of the photosphere in late November 2021. Figure 1 summarizes the plasma10, energetic ion11 and magnetic field measurements12 made near perihelion during E10. A spectrogram of ions in Fig. 1a and 1b extends from thermal energies to ~85 keV and, like the proton velocity in Fig. 1c, is structured as discrete ‘microstreams’13,14,8 whose duration decreases from ~10 hours to ~2 hours as the spacecraft approaches perihelion on 08:25 Nov 21, 2021. Data presented in Fig. 4b (and discussed later in greater detail) reveals that the wind ion energy distributions are power laws at high energy that extend to greater than 100 keV. The characteristic structure of the microstreams is highlighted by red arcs in Fig. 1c and a blue trace indicates the measured thermal alpha particle abundance AHe = nα/np which is similarly modulated. The high First Ionization Potential (FIP) of Helium requires that the alpha particle abundance is frozen-in at very low altitudes in the corona or in the chromosphere at the base of the wind15, so these microstream structures are organized at the source of the wind itself. The radial component of the measured interplanetary magnetic field in Fig. 1d shows that large-amplitude, Alfvenic field reversals, that have been termed ‘switchbacks’, are also associated with the microstreams. A Potential Field Source Surface (PFSS) model16-18 [see Supplementary Information] of the coronal magnetic field is used to infer the footpoints of the magnetic field that connects to the PSP spacecraft and reveals the connection to two separate near-equatorial coronal holes. The time series of the longitude of the footpoint on the solar surface is shown in Fig. 1e and as white diamonds against a 193Å SDO/EUV19 image in Figure 2.
The linkage of the temporal structure of the switchback and radial velocity bursts with the spatial periodicity of the surface magnetic field documented in Figs. 1 and 2 suggests the possibility that magnetic reconnection between open and closed magnetic fields in the low corona (interchange reconnection) is the driver of these bursts8. Consistent with the observations, interchange reconnection in the weakly collisional corona is expected to be bursty rather than steady20-23. The energetic ions and enhanced pressure in these burst intervals are also signatures of reconnection24-26 and further support the reconnection hypothesis as the source of these bursts. If reconnection is the driver of these bursts, the data suggests that interchange reconnection is a continuous process in the source regions of open flux. Figure 3c is a schematic (in 2D for simplicity) that shows open flux reconnecting with closed flux regions in the low corona (see the caption for a detailed description). In this figure the open flux migrates to the left, reconnecting with successive regions of closed flux with the consequence that the bursty outflow from interchange reconnection fills all of the open flux as seen in the data.
Interchange Reconnection
To establish that interchange reconnection is the source of the bursty radial flows, we use the observational data combined with well-established principles of reconnection to deduce the basic characteristics of reconnection in the low corona. The strength of the magnetic field undergoing reconnection is a key parameter. Since the magnetic field strength at the base of the corona has significant variation, we estimate the amplitude of the reconnecting magnetic field by projecting the measured magnetic field at PSP back to the solar surface. The usual R-2 falloff of the radial magnetic field with radius R is valid in the solar wind, but fails closer to the Sun. Thus, we use a combination of the R-2 behavior at large R with a falloff derived from a surface averaged PFSS model below 2.5 Rs (see Supplementary Material.) The resulting projection of the 600 nT magnetic field at 13.4 Rs to the low corona is 4.5 G, which is consistent with the PFSS data in Fig. 2.
The plasma density undergoing reconnection at the base of the corona is not measured directly. However, the characteristic amplitude of the bursty flows at PSP are around 300km/s. Since the flows during bursty reconnection are Alfvenic, we can estimate the density knowing the magnetic field strength. The resulting density is around ~109/cm3, which is again a reasonable value for the low corona27.
To address whether the rate of energy release during reconnection is sufficient to drive the wind requires an estimate for the reconnection inflow rate Vr. A lower limit for the rate follows from the fact that the flow bursts are nearly continuous. To see this, we define the reconnection time tr = LB/Vr, the time required for open field lines to traverse the characteristic scale length LB of the surface magnetic field, which is around 10o or 6×104 km. A second time is the time tb ~ RPSP/VR for the reconnection bursts to reach the spacecraft at RPSP. In the limit tr >> tb, the outflows from the reconnection site in the corona would quickly pass by the spacecraft and there would be no high-speed flows until the spacecraft connected to another reconnection site in the corona. When tr ≤ tb, the spacecraft would measure bursty flows as the spacecraft crossed the entire supergranulation scale. The observations reveal the latter since bursty flows are measured during the entire crossing of the supergranulation scale. Thus, the observations suggest that tr ~ tb or Vr ~ LBVR/RPSP ~ 30 km/s or around 0.1 of the local Alfven speed, a reasonable value if reconnection is collisionless28-30 but somewhat faster than the MHD prediction31. However, for the parameters of reconnection just calculated and with ambient temperatures of around 100 eV the reconnection electric field is around four orders of magnitude above the Dreicer runaway field. In this regime classical collisions are too weak to limit electron acceleration and collisionless processes dominate. Interchange reconnection driving the bursty flows measured by PSP is therefore collisionless and the rate of reconnection is consistent with expectations.
The rate of magnetic energy release from interchange reconnection is given by VrB2/4π ~ 5×105 ergs/cm2sec using B = 4.5 G and Vr = 30 km/s. This is comparable to that required to drive the high speed wind, which is around 105–106 ergs/cm2sec1.
Thus, we have established through the PSP observations, the SDO/HMI surface magnetic field measurements and well-established characteristics of magnetic reconnection that interchange reconnection is sufficient to drive both the ambient base solar wind flow (through the pressure increase of the ambient plasma) as well as the strong magnetically driven flow bursts that lie on top of this flow. Specifically, the base solar wind is driven by the usual radial pressure drop1 as well as the exospheric processes32 associated with strong electron heating. Further tests of the interchange reconnection explanation of the microstream structure of the wind measured at PSP concerns the structuring of the flow bursts and the production of energetic protons and alphas. A key observation reported in the E06 data8 and illustrated in the schematic in Fig. 3c is the time asymmetry in the burst amplitudes: large amplitude bursts onset sharply and gradually decrease in amplitude across the burst period and the time sequence then repeats. Data from a particle-in-cell (PIC) simulation of interchange reconnection is presented in Fig. 3b (See also the Supplementary material). A cut across the outflow simulation exhaust reveals high speed bursts on newly reconnected field lines in the exhaust adjacent to the magnetic separatrix while on field lines in the exhaust interior, the fastest flow bursts have already passed the location of the cut so the measured flows are weaker (Figure 3a). Thus, the simulations support the hypothesis that the bursts observed by PSP correspond to crossings of interchange reconnection exhausts. Such dispersion signatures have been well documented in the cusp region of the Earth’s magnetosphere as a result of reconnection at the terrestrial magnetopause33. Reconnection between the closed magnetic flux of the Earth and “open” flux in the solar wind is a clear analogue of coronal interchange reconnection.
Finally, the spectrum of energetic protons and alphas has been calculated from the interchange reconnection simulations. The simulation includes fully stripped alpha particles that are 5% by number, close to that expected in the solar atmosphere15. The spectrum of the energy flux of both species is shown in Fig. 4a. This data is taken from the outflow exhaust and so includes only plasma that has undergone acceleration either near the x-line or on entry into the exhaust. Both the protons and alphas exhibit an energetic, non-thermal powerlaw distribution with spectral indices of around -7 for both species. As shown in Fig. 4b, the spectrum of the differential energy flux of particles during the time interval 04:00-19:00 Nov 20, 2021 from Fig. 1, there are also energetic protons and alphas during the switchback bursts with energies up to around 100 keV. The spectra are again rather soft, having spectral indices of around -7, consistent with the simulation data. The energy in the simulation is normalized to the free parameter miVA2. By equating the energy minimum of the proton powerlaw in the simulation (~3.6 miVA2) to that of the SPAN data (~5 keV), we find that the coronal value of miVA2 is around 1.4 keV compared with around 0.9 keV from the 300 km/s estimate for VA based on the amplitude of the bursty flows measured by SPAN at 13.4 Rs. That the two values of miVA2 are close indicates that the Alfven speed in the corona where reconnection is taking place is in the range of 300 to 400 km/s.