**2. The local interstellar medium (LISM)**

The LISM encloses the plasma region generated by the Sun in what is known as solar wind (SW), plus its shocked plasma region known as the heliosphere sheath. The interstellar medium, which its in-situ known region we call the very LISM is of uncertain origin. Some workers attribute its formation to be the wake of a supernova that occurred millions of years ago. This region is known as the local interstellar molecular cloud, of which **Figure 1** shows a sketch, and an artistic representation is shown in **Figure 2**.

This interstellar molecular cloud is in the path of the solar system, which by now and from hundreds of thousands of years has been immersed in it. The very LISM is then the region of the molecular cloud modified/surrounding the plasma pushing out of the Sun and the LISM region we explore with SC Voyager since the year 2012 with Voyager 1 (V1), and 2018 with Voyager 2 (V2), see, e.g. Berdichevsky [7] on the sheath region before the arrival of the SC to the heliopause.

**Figures 1** and **2** identifiers (e.g. representation of the star Sun) are not at scale, but the distances are, and in the bottom right, it is indicated a 'parsec as the reference unit giving an idea of the dimensions involved in the sketch. The Voyager 1, currently at <sup>7</sup><sup>10</sup><sup>4</sup> parsec, in several years will arrive at 0.001 parsecs from the Sun at their velocity as it moves, escaping the gravitational field attraction of the solar system.

#### **Figure 1.**

*A sketch, and plane of sky projection of the local molecular cloud including location of the Sun and other stars.*

*Hydromagnetic Steady Magnetized Plasma Encountered by Voyager in the Interstellar Space DOI: http://dx.doi.org/10.5772/intechopen.112362*

#### **Figure 2.** *An artistic representation by Linda huff and Priscilla Frisch.*

Observed in the interstellar medium, since its entrance in 2012 by V1, see the announcement by NASA [1], see **Figure 3**, are the large intensity of the **B**-field, and the four times occurrence of a compression process in which changes in **B-**field magnitude coincide with the change of density observed, [8]. Good measurements of the plasma in the LISM have been achieved using the radio instrument in both V1 and V2 [5]. On the other hand, the plasma instrument design to measure properties of the

SW, [3], is not optimal for identifying the LISM plasma properties, but they achieve reasonable evaluations with large errors in V2 (in V1, that instrument failed a long time ago).

The solar system displaces relative to the LISM at solely a few tens of kms<sup>1</sup> . At such speed, no shock at the heliopause interface is expected nor it has been encountered. Here, we assume a shock in magnetic field frozen matter (see, e.g. [9]) that had been identified to be consistent with a process called collision-less [10]. However, there are some workers in the field who brought the idea of shocks in the LISM medium that, in their view, are collisional, and that is a current subject of debate (see,

*Hydromagnetic Steady Magnetized Plasma Encountered by Voyager in the Interstellar Space DOI: http://dx.doi.org/10.5772/intechopen.112362*

**Figure 5.**

*The orientation of B-field observed both by V1 and V2 outside heliopause (hp).*

e.g. [11]). Then at their suggestions in **Figure 3**, there are compressions labeled as shocks and others just compressions. We will come to the subject of these types of compressions later.

The global representations of the interface between heliosphere and very LISM are shown in **Figure 4**, top panel [12] and bottom panel [13, 14] are competing interpretations which currently dominate the debate on the subject. In the bottom panel, the meridional cut of a 3-D representation is indicated, different main regions of the magnetosphere as well as the location of the V1 and V2 SC relative to a reference astrophysical coordinate system where the top is pointed in a direction which approximately is defined by our heliosphere standard North definition.

**Figure 3**, in [8], shows that V1 encounters four compression jumps in which density and magnetic field data indicate an equal value for the change, both in the magnitude of the **B-**field as well as in the plasma density content. Based on that fact and also the orientation of the LISM **B-**field wrapped around the heliosphere plasma with its expansion stopped (**Figure 5**) at the heliopause (hp), and which are well identified, we make the educated assumption that the medium possesses the properties of a simple ideal MHD state, see, e.g. [15]. This subject introduces us to the next section.
