Welcome to... "Micro-Lesson 241"
This riff uses the open 5th, "A," string to promote the tonal center of, "A." The Dorian relationship enters by way of the use of the appearance of the major IV-chord, (D Major).
However, the use of an, "F# Minor," double-stop creates an interesting tie to the Natural Minor tonality as well. The application of this sound as a, "Double-Stop," unfortunately does not suggest enough harmony to fully commit to that Natural Minor mode.
In measure one, double-stops are applied from off of the, "A," tone from open 5th string. This open tone is played against the sound of, "G Major," and heads down the neck across an, "F#m," double-stop and an, "A Powerchord."
The, "Csus2," in measure two appears on measure one's up-beat of the count of four and pulls in the, "A" Powerchord once again to anchor the key center.
Measure three brings back the riff from measure one but it changes up at the tail end where the, "G," root of a, "G," Powerchord pushes in our fourth bar.
In measure four we arpeggiate the, "G" Powerchord and the, "D Major," chord (which uses an "F#" in the bass). This bar is what truly anchors the effects of the, "A Dorian Mode." Enjoy!
Micro Lesson 241: "A Dorian" Classic Rock Riff